At our program the night before is supposed to be informal. Heck we hold it at a bar and only the residents and applicants go (with the chairman's credit card!). We expect all the questions that you don't want to ask in front of others (e.g. what's the call schedule really like, is the dating scene tolerable, etc. etc.) There is no formal reporting system to link the information garnered at the bar to the interview file, however, if a resident feels strongly about someone (positive or negative) he or she may let the education office know and that will get factored in. I guess what I'm saying is that, unless you make a complete jerk of yourself, it wouldn't make a difference at our program.
However, I also know of places where the night before is a quite formal event with attendings, etc. I would take those ultra-seriously as another interview as it is very very likely that you will interact with people who will have an impact on your candidacy.
As always YMMV (Your Mileage May Vary). Bottom line, act as if you were meeting your significant other's parents for the first time and you'll be fine. As far as dress, the suggestion of asking the office before you come is probably your safest bet.
