Speaking of simsubs, as we were:
There are two paths here, one going to agents, one going to editors.
If you're looking for an agent, it's normal and expected to query a dozen or fifty at a time. Just spell their names right. If one comes back and asks for an exclusive, make sure you have reasonable time limits and dates on it. A six week exclusive isn't out of bounds.
I keep hearing, "Suppose I hear back from a better agent?" The answer, O seeker after wisdom, is this: Don't query any agent you wouldn't be delighted to have represent you.
The second path is through editors. Here, only simsub if the market explicitly states that it's okay. If they're silent on the subject, assume no simsubs. If they say "No simultaneous submissions," it would behoove you to believe them.
Now on to cases. Suppose you submit to a bunch markets that allow simsubs. Suppose you get an offer back from a 1/4-cent-a-word market, you gleefully accept it, and the next day you get an offer from a ten-cent-a-word market. What then, Pilgrim? Answer: Same as above, don't submit to any market you wouldn't be delighted to have publish you.
You will find folks who say, "Go on, young writer. Submit simultaneously to markets that say 'No simultaneous submissions.' You're only hurting yourself by giving 'em exclusive looks."
I say, "Bah! Humbug!" You don't win a prize for getting the most rejections soonest. You're working on your next piece.
First, that's a form of betting against yourself. You're betting that no one will want your story anyway.
I will tell you a true thing: A story that's publishable by one is publishable by many. If you're writing at a publishable level, you're likely to get more than one offer. What then, sprout?
By the time an offer comes to you, the publisher has already expended time and money on your piece. They've run profit/loss numbers, figured out where it'll fit in the schedule, and are ready to go with it. They won't be happy to have it withdrawn.
Next, you will be found out. Editors all know each other. They talk with each other. One of the things they talk about is the great new writer they just discovered. So if your story is that flaming good (and why did you submit it if it wasn't?), they're going to be talking about it with their friends from other houses while they're picking up their sandwiches to eat back at their desks. (If al Qaeda wanted to destroy New York publishing there's one particular deli they could bomb at lunchtime.)
One of the fictions that you're writing is that the publisher you submitted to is the one among all the publishers in the world that you really, really want to see publish your book. (That's why in the cover letter you want to make sure that not only did you spell the name of the editor correctly, but that the editor works there and you've changed all references to the publishing house to the name of the house you're currently submitting to.) They want to think that they're the first girl you asked to the prom.
So, why not anyway? Because the next time one of your books comes in, the folks who see it will say "Bet he's submitted it to everyone in Writer's Market" and slip it back in the SASE with a pre-printed form. Faced with 18,000 slush manuscripts, editors are looking for easy rejects. "Functionally illiterate from Page One" is good for that, but "Doesn't follow the guidelines" is also fast and easy. Editors aren't cutting you any slack; they're looking for reasons to say no.
Shall we talk about agents and auctions now?
Those are one reason it's good to get an agent. Agents aren't limited to one submission at a time. They can hold auctions, which is a form of simultaneous submission. They are banking their reputations on finding Good Stuff -- so you need an agent with that reputation.
The agent calls up however-many of her editor friends who might like the manuscript, and says "I'm holding an auction ... do you want in?" and messengers the manuscript over to the ones who say yes. It's got a closing date and time. After that, the fun starts.
The first publisher to come in with a reasonable offer -- one that the author would accept -- gets the floor. If no other acceptable bids come in, they get the book. (There are advantages to having the floor, which I'll get to in a minute.) If other bids come in, all the folks who are bidding are informed, and can come back with better offers. Better offers may not be for more money -- they may be for future books, or variations in rights sought, or accounting, or publicity.... and so on, until the auction closes. At that point, the publisher that got the floor gets a chance to trump whatever the winning bit was, by paying 10% more. So whoever gets in the first bid is guaranteed to get the book, if they want it enough.
That's where you want a savvy agent.