Most supervisors will not have dressed up to interview you and will not expect you to. However there's no accounting for taste so dressing smartly is the safest bet. It's probably best not to go over the top, but if your choice is between a suit or crotch worn Levis with a pair of espadrilles and Marilyn Manson T-Shirt, then just put the suit on and don't worry about it - you can save the Levis for your first day.