Everybody's different. I drink quite heavily almost every weekend. Sometimes middle of the week. I've got an extremely high tolerance for alcohol, so when I drink, I drink a lot.
The biggest concern in my experience is not letting the lowered inhibitions, kill your diet. If you go out drinking, and then go to Waffle House, you're toast. But if you make sure that you cheating on your diet while you're boozing is not acceptable, you're going to be fine.
Granted, I'm not talking about competitors here. I'm talking about real, everyday guys, who work real jobs and also love to crush iron in the gym. It's what you do "most" of the time that matters. So if you're getting shit-faced 2 nights a week, then out of the 168 hours in that week, you're intoxicated for 12-20 hours (7% - 12%), leaving 88% of the time to be completely committed. 88% is pretty f#cking good in my book!
Also, nothing beats a hangover like a lot of water and a good workout!