It's a delicate balance for me. I make more mistakes when I drink, but I actually perform better overall, including stage presence, posing etc.
It's still a max of 3 pints until I've finished the playing though.
When I used to play sax with a motown/blues brothers type band I could drink as much as I wanted and still play fine. I find guitar technically more challenging though, hence the difference.
I think it takes a bit of trial and error to find out what's the best balance for each person.