I would say drink after the performance. If you are being paid to perform, it is a job like any other and drinking on the job is unprofessional. The bar setting can be a temptaion to drink, but not the best idea. I am a bartender and musician, and although bartenders may be tempted to have a drink while socializing with the patrons, and one or two won't make you intoxicated, it is not exactly legal to drink on the job. Alcohol impairs your judgement, and for bartenders, if your judgement is impaired, you shouldn't be serving patrons for many reasons (ie knowing when a patron has been overserved, handling cash, etc.) The same should be said for any other profession.