The only time I didn't send five pages was if they said "query will be deleted unread if it includes pages." Or I suppose if there was big flashing signs around "only" the query."
It made a big difference for me. The first time, most of my queries didn't get pages because most people didn't ask for them. They generally said send a query and that was it, so that's what I sent.
Janet Reid is the one I read the advice from to send 5 pages each time, and so the second book I queried I sent pages with EVERYTHING. My request rate went way up. My opening pages were pretty rocking, so I was pretty sure I could get anyone who was on the fence on board if they read any of the pages. And that's the point. Someone who is "meh" about the query will just say no without pages. If they look down and your pages are really good, though, they might be willing to take a look.
You have to make sure your pages are amazing, though. No mistakes, clunky writing, etc. But yes, I would recommend doing it. The first time I followed the "rules" to the letter each time. The second time I broke them all over the place. I wouldn't have an agent if I hadn't. You just have to be smart about how you do it. Sending fantasy to an agent who doesn't accept it is a bad idea, and sending three chapters to someone who says to send a query only is flauntingly disobeying instructions, and not sending synopses when requested is silly, etc.
It's your book and I can understand wanting to follow the letter of the law, but in my experience the letter of the law is a little more like guidelines, and as long as you aren't stupid, bending rules sometimes is very worth it.