I'd pretty much recommend the default of "no flashbacks," and make sure there's no other way to present the backstory otherwise before using one. And I should say that by "flashback," I'm talking about a full-on, scene-length chunk of text, not just a sentence along the lines of "he'd always been a good kisser, even back before he'd gone to med school and studied anatomy, specializing in erogenous zones," which is fine as long as it's not an infodump or authorial intrusion, but is actually something that the pov character would actually think at the time.
The reason I hear most often to justify a flashback is, "the reader needs to know what happened X years/months/days ago, so she'll understand the rest of the story."
Two problems with that. First, what the author is saying, really, is that the AUTHOR needs the reader to know what happened then, which is slightly different from the reader needing to know what happened. It's author-driven, rather than reader-driven. A lot of the time, this is just an easy out for the author, and it's bad for the reader, because the past event is being dumped on the reader (a sort of infodump in disguise), and, even worse, it's killing any tension that might arise from the reader wondering, "gee, I wonder what happened in the past that made him/her act this way, and I'll have to keep reading to find out," b/c the answer is right there, often early in the story. OTOH, if the reader really and truly needs to know what happened, in blow by blow, full-scene detail, and would be thinking at that point in the story, "I really need to know every detail of what happened back then, and I need to know right this minute," then go ahead and do the flashback. But if the reason is that the author wants the reader to know something, then Just Don't Do It.
Second problem -- in the vast majority of samples I see, the details of what actually happened in the past don't really matter. The past event really is backstory and explains motivation and characterization, and it's not part of the actual story. It doesn't so much matter what exact words were said or actions were taken (all the stuff that would go in a flashback), as how the point of view character remembers them now, or how the pov character feels about them now. What matters isn't stuff like "A said this and B said that and A did this and B did that and they were in a particular location and the weather was cold and it was afternoon." What matters is that, b/c of that event, the character has a particular bit of emotional baggage now. The basic outline of what happened is all the reader cares about, not all the details that a full-fledged flashback entails. And if that's true, then there are usually much more engaging ways to present the information.
Just as an example, and using romance, since that's the genre I know best, and using a teeny-weeny scene of the sort that's often believed to be necessary -- the flashback to when the h/h first met briefly years ago.
Flashback:
It was a dark and stormy night. Hannah Heroine was standing beneath a tree, trying not to get electrocuted, belatedly remembering that the correct action for a storm was to stay low to the ground, far away from trees.
She dropped to her knees, scraping them, but figuring it was better than frying her whole body. She flopped forward and snaked across the rocky, weedy field toward the highway, where she hoped to hitch a ride home.
In what should have been silence after a too-close lightning strike, she heard someone else swishing toward her. She froze.
A moment later, a hand brushed her foot. Panicky, she began moving again, but she was no snake and she couldn't stand up and run as long as the lightening was a threat.
"Slow down," came a male voice behind her. "I've got a tarp and I'm willing to share."
[Etc. Flashback scene ends with them kissing, which is interrupted when they're rescued by a passing police cruiser.]
Okay, so now we know she was once caught in a lightning storm. She met the hero in a lightning storm. They kissed during the storm. But: so what? The event wasn't really that interesting in its own right. And it could have a gazillion different consequences today, all of which are more interesting than that mini-scene. Especially when the real story is happening twenty years later, and Hannah's a different person than she was back then.
But, see, that's where it can get interesting if the author knows that event happened, even if the reader doesn't, especially if the reader doesn't, because then the past event can affect current actions, which are the ones the readers care about. After all, one possibility is that Hannah went home, got warm, got drunk and never even remembered the storm or the kiss. That event doesn't affect who she is today at all. Another possibility is that she was totally traumatized and associates kissing with the possibility of dying, so she's become a nun. Another possibility is that she's become self-sufficient in many ways, but succumbs to wild bouts of lust during storms.
Let's say she's the last example. In that case, a more interesting way of revealing the past, to the extent it even matters, is through little details that add up to characterize her. Hannah always carries a tarp in her purse. A little one, but still, it's always, always there and always with her, whether she's flying across country or just walking down the hall from her apartment to pick up her mail. And that little detail comes out in a scene, and the reader thinks, "huh, that's interesting. I wonder why she is so obsessed with tarps, and I'll have to keep reading to find out," and then in another scene, there's lightning, and she gets a bit aroused, looking for a guy to kiss, and the reader connects it with the tarp and suspects there was an event involving tarps and kissing and storms in the past. Or Hannah's with the hero and a storm is imminent, and she can't wait (or she hopes it passes by, if she's resisting him) b/c she knows she always get a little hormonal whenever there's lightning. And eventually she'll say something like that, or remind him that they met during a lightning storm, or someone else will say that she's been like that ever since she was caught in a storm. Or whatever. And the reader will think, "Aha! I get it now" or "Aha! I thought it was something like that." But it'll just be the key bit of information, the thing that stuck with Hannah as time passed and that made her who she is. Not every little bit of what happened. Not the rocks and the snaking and the exact dialogue. Just the residual effects.
Heck, what happened and how she remembered it may even be two totally different things, which is yet another reason why flashbacks are troublesome unless you want to have an unreliable narrator situation: she remembers it as scary and gross and painful until the kiss itself. He remembers it as a challenge, up to and including the kiss. She remembers really bad lightning, he remembers a distant strike or two. Doesn't really matter which one is "right." What matters is how it affects them in the story now.
So, bottom line for me: Stick to the story's "now," unless you've got a really, really pressing need to go back, and it's a need that the reader will feel, not just the author. Then, if you go back, make sure a) the flashback scene is interesting in its own right, complete with conflict and rising tension, and b) the timing is right, so you haven't killed all the story tension.
JD, rambling a bit; it's been a long day