Signed on the back of the headstock or on the control plate is ok. Never on the front of the body no matter who it is! Front of the headstock is acceptable but not preferred..
I personally do not like the chore of protecting a signature on an instrument though. I would rather have a different memento signed for posterity in most cases.
In my youth, I went to a LOT of shows and there was this guy that would bring the same red Ibanez to every show to get signed. He was also missing a leg, so he would use the guitar and the sympathy factor with the crew to meet the bands. The sigs were always removed before the next show would roll around. Funny thing is, 9 times outta 10 if he met who he was after and got a handful of guitar picks, a couple of autographs and a snapshot with Joe Rockstar, he wouldnt even stick around for the show. Never understood the fascination with the artist if you aren't there for the music, what's the point?