Watching a show? Depends. If it's close enough for me to walk or if I've got a ride, maybe then I'll get good and hammered, but not if I have to drive.

Playing a show? I hate playing drunk, so I might have a drink or two but no more.

I can't imagine a worse thing than playing a show and thinking you were rocking the place only to have your friends tell you "dude, you sucked last night."