Even better, don't. This is (or at least was) discussed occasionally, typically with context 'that sounds like ass, and the old guy at the board is just going to adjust the vocal track to make it sound slightly less like ass and as a result uncupped sections will sound all wonky and confused, so don't do it you stupid git'. It's an absolute bastard to get that working properly, and doing it improperly is just going to sound stupid.
Jeff
