Again, I have to echo many sentiments here and dress in at least business casual.
If I'm an examiner and someone shows up looking like they rolled out of bed, I'm not going to think too highly of that person. In fact, I may be more inclined to find something to fail them on. I'm also thinking, "this person doesn't have enough sense to take care of himself then (s)he may be sloppy in their airwork."
Dressing nicely also shows you take your role as a pilot seriously, be it a weekend warrior or a full-time professional pilot. To that end who would you be more comfortable putting your family with: the guy who dresses in a button-down shirt with Dockers or the guy in in the ripped up sweatshirt and stained jeans?
While we're on the subject, there's a small airline here called the Nantucket Shuttle. They're basically a discount competitor to Island Air and Cape Air. They're so discount that their pilots don't wear a uniform. I saw one Captain wearing exactly what I described above (the ripped up, inside-out sweatshirt and torn, stained jeans) and had a beard to make Johnny Damon look professional. Needless to say that I have stayed far away from that operation.