As a former CM with friends who still work there, I can assure everyone that seating requests are no big deal at all! It's all sort of a game of Tetris anyway...depending on the ride you might need weight distribution, or to direct larger riders to specific seats, or to try and place parties to reduce the number of empty seats (especially if there's no single rider line). So a guest request is just one more piece of the mental puzzle we're doing for every boarding group anyway. And if we have to have you wait till the next ride cycle, in a way that's made our job easier, because it gives us an anchor point to start that boarding group (the other big anchor point is wheelchairs/transfers, because they can only go in certain spots).
Definitely be prepared to wait a cycle or two, but CMs will generally let you know if there's already a line for the seat you requested and about how long the wait will be. And there are occasional instances where the request might not be possible (for example, if you're on an
ECV, B1 of Soarin' is impossible, because
ECVs can't make the tight turn...at least, that was the case when my dad was using an ECV a few years ago). And of course, anyone can have a bad day, including CMs, so you might run into someone grumpy. But honestly, for 99.99% of situations and 99.99% of CMs, we want you to sit where you want to sit, and it's no big deal at all for us, so request away!
ETA: Just don't do what some guests do and decide to wander into the row you'd like rather than the one we assigned. That's a first-class ticket to a grumpy CM, as well as grumpy guests whose seats you've now taken. Always request before you're assigned a row.