I don't drink at all anymore or gig for that matter. When I was gigging in college (and living off the money I made gigging) I drank excessively before, during and after a show. I did play most gigs in college towns where all of the audience was also drinking. I don't believe it hindered my performance and I have a bass playing buddy who will say they same thing. He saw me some nights were I could barely stand but still never missed a note. When I left that band, I joined his as a guitar player. I remember our first gig, the whole band was sitting at a booth at the bar and he told the band no drinking before the show. When the rest of the band went to set up, I asked him about that and he said I could drink all I wanted.
In the early 2000's I was in a few bands and I wasn't drinking then, so I didn't drink at gigs. I think as long as it doesn't have a bad impact on the performance and I it is a gig where folks are drinking, it is fine. Don't start doing shots if you are playing an AA function.