If I am really tense before a show (you know, life wearing me down kind of thing), I will have one beer. Maybe two. Also, if someone wants to buy the band shots during the show, I'll take one...but only one. I'm a big guy, so one shot or two beers won't really have an effect.
However, I'm impressed by the responses in here. I talk to musicians all the time who swear they play better drunk. When I point out that they are just wrong, and that they only think they play better because they are drunk, so everything feels better, they don't buy it.