Actually, I'd prefer to be perfectly dressed, but wouldn't we all? Since that isn't an option, I'll have to say overdressed. If you're overdressed, you will stand out, but likely in a positive way. If, however, you show up to a black tie affair in casual clothes, there isn't much you can say or do not to make a negative impression.

Of course, there are the extremes of being either. Examples would be showing up to a frat party in a ball gown or showing up to a ball in jeans and a t-shirt. You have to err on the side of caution when you're not sure what to wear, but being slightly overdressed is always preferable, in my opinion.

Looking your absolute best at all times in social situations gives an impression that you care about yourself and what others think of you. If you seem confident enough to dress up a little for casual affair, and not let it bother you, you will definitely draw more positive than negative attention, which will boost your self-confidence, and make you more pleasant to be around. Being comfortable with yourself, what you wear, and who you are is very sexy and appealing.