All of the travel sites will tell you that dressing better makes it much more likely that you will get treated well by the airline if something goes wrong. You don't want to be arguing about compensation for a cancellation in your pikachu pajama bottoms and "America F*ck Yeah!" sweatshirt. No need to wear a suit and tie, but nice jeans or chinos, an OCBD or nice polo and a sportcoat is good. The sportcoat will go in the overhead and you are just as comfortable as in sweats (OK, nearly), but you'll feel better about yourself and get the advantage of better treatment by the airline.

Just my $.02