It's not really that common, and I'm not sure it would help you in the eyes of adcoms. But it could be helpful for you.
Still, I doubt it would hurt to send a quick email to the admissions people and say you are interested and ask if you could stop by and ask some questions. Maybe they would help you out, maybe they wouldn't. No harm in asking though. I would do this before you go. I wouldn't just drop in.
If that doesn't work, you can use any connections you have or even SDN to track down some current students and get an informal tour/information session. Med students always could use a free lunch or some coffee.