Heck, I've taken one before a comp heat. There is definitely a balance point. A little can help your focus by relaxing you and getting unrelated matters out of your head. But too much reduces your sensitivity and slows your reaction time, and then everything gets out of whack with your partner.
Heh. This reminds me of a social dance I went to a few years back. It was margarita night, and I had one (emphasis on the one) during the mid-dance break. The first dance after the break was a quickstep, which my am-partner-at-the-time and I always danced together. About halfway down the first long wall, I got a "how many have you had?!" from her.
Suffice it to say, I've avoided pre-competition alcohol.
Yep...lol get this...first time dancing with this guy and his manners need an adjustment. During post lesson practice he decides to ask me to dance, quiries me what dances and I know, ignores my reply then proceeds to fling me into some random freestyle stuff I am supposed to magically acclimate to as a newbie then dips me. Then stops mid dance and asks me "how much do you weigh?" Just hand in your "man-card"...you never ask a lady how much she weighs! Wtf?
I think a good rule of thumb can be that if someone makes you feel uncomfortable don't spend more time with them. Of course sometimes you can't avoid it. I have had some funny situations at the Chinese studio where I have worked. There, they will actually just skip the person if they don't like them and no explanation is required.