Dress so as to look "Classy" at the venue, unless there really are points to be scored for fitting into the scene. Generally you have to be at least as smart as the punters, prefeably a little more so. You want to make it clear that you are present as a professional entertainer. Don't go too far however, as this can be counterproductive (look at him, all dressed up....)

Speaking personally, I dress up a bit even for private family do's. Indeed, my (musical) partner and I are regularly told that we "scrub up well". I find if I haven't dressed for the occasion, I cannot perform at my best - it just doesn't feel the same.