Is it a good idea to get a little bit pissed before you play onstage?

We've got to play four or five songs this week for a 'farewell' concert to our doomed school, and its going to be a really bad audience - probably just suits and parents with about 20 of our mates.

Can you think of any disadvantages to getting slightly pissed before going on? I don't mean passing-out drunk or anything, just as much as would get you dancing at a party.