I think the point with the business suit is that it is better to play it safe. If you are 100% sure that the interview does not require one then there is no need for it. But I would definitely wear one if you are unsure.

Think about it this way - if the judging panel did not expect you to dress up but you did it anyway then no big deal, but if they did and you are on your flannels and jeans then your experience is going to be quite uncomfortable.

No matter how nerdy you are, having a bit of fashion sense in interviews gives an automatic outward aromatic presence - very critical in business graduate school interviews, perhaps not so much for the arts and sciences.