I may take a puff or two beforehand (a band or two before going on). I know my limits. I stay hydrated before and during (H2O or Gatoraide) as our shows are quite physical. I don't drink too much alcohol, a beer before going on and one onstage would be an exageration. Generally hydration before/during the show and a Jagerbomb onstage, then hydration afterwords.
Now my drummer on the otherhand...We had a show lastnight. It's our drummers birthday this Monday and he wanted to get his drink on, not that it is very unusual as he is a HUGE Jack and Coke fan. Regardless, any one gig it wouldn't be unusual for him to have had a few before going on stage and he can perform flawlessly. HOWEVER, when he really is determined to drink, well, lets just say the show lastnight took us to different places often throughout the entire show. He also said that he really couldn't hear our vocalist too well as he had no monitor.
It was a bad show in general, our guitar player broke a string on the first song (new strings only one or two gigs in (he NEVER brakes strings)) then about three songs until the end of the set, the back screw of my strap strips out!!! That's never happened to me on this bass.
Still, a decent crowd, a good guarantee, another late night.
As a band, there are occational exeptions, such as lastnight; a hometown crowd that knew this was a little party for our drummer, many of which probably didn't even catch many of our drummers misdirection. In general however, "F" ing up isn't tolerated. You cannot be wasted and perform professionally. If you have guarantees, sponsorships, endorsements, fans, you owe them respect and you must carry yourself in a professional manner. People expect to be entertained and it is your job to entertain. If your not that serious that's fine, don't keep your expectation too high, but if your trying to move onword to bigger and better gigs, "party everynight" doesn't cut it IMO.