I usually wear whatever im wearing that day. Have shown up to meetings in formal wear (suit & tie), casual wear (jeans and t shirt), lounge wear (not pyjama kind) and never noticed a difference in service based on what i was wearing.
Only time i ever got a reaction was when i did not have time to go home and change and show up in full motorcycle gear (one piece leather suit, long gloves and boots, not the S&M kind) but it didnt affect the service once she got passed the initial WTF stage and figured out im not crazy.
Hygiene will have a much bigger impact on your encounter than your looks. If you want to put extra effort on something to prepare for your date I would suggest taking all possible measures to prevent bad breath and BO.