Even if Apple releases iOS 6 over OTA, iOS 5 users should perform full backup in iTunes first, followed by update.The real question. Will GM be OTA?
It wasn't with iOS5... but then again OTA was a new, beta'ish feature at that time.
I'm not counting on it. But would be nice.
As for developers running iOS 6 beta, it's probably best to perform full upgrade to get rid of any beta-ness anyway.