I find sun glasses and a cap don't hide my trembling hands on a big call...or my voluntarily drawing in of breath on a bad flop when I bet big pre-flop.
I agree there are a lot of jokers out there who where sun glasses and hats just because they think they look better but if you believe it helps your game then do it. That's all that matters.
I agree with Etre. If you believe it helps your game then wear what you like. HOWEVER i believe sponsorship is teh best reason to wear caps etc. That is if you are lucky enough to get sponsored.