I had braces for a year, after which the dentist put in a little metal retainer behind the back of my lower arch. With that visit, he tells me that, with my wisdom teeth growing out and all, there's a chance that it could undo all the work my braces did. That, and, with finals and such (Ie., times of stress) they'd be painful in the process. Finishing off, he tells me, not even suggests, to make an appointment with his receptionist to have them taken out. I never went back.

*pokes at the one and a half year old metal strut* Wisdom teeth grew out without any pain whatsoever, though he was partly right about the undoing bit - one tooth has moved partly back to where it was. Still, it's invisible unless you actually look for it, so it's all good.