At every single medical school interview I've been to, I've been told numerous times that appearance does not make a difference whatsoever to your interview performance, and that the interviewers are specifically trained to only judge you on what you say.
However, it is impossible to stop the thoughts rolling in the interviewers' heads. You can't really stop the immediate bias from interviewers if you dress and look like a total ass at an interview. For all the med interviews I've been to, adelaide, unsw, newcastle, monash, griffith and uwa, 99% of them dressed very formally. If a guy suddenly walked in, with baggy jeans, half torn shirts and piercings up their nose and ears and eyebrows, it probably isn't much of a good impression to start with. I'd say go for the 'good' looks for the day. Removing the piercings and such...