If you care about your appearance and wan't to promote a proffesional aspect you will take pride in what you wear. Felling good about yourself also creates an air of confidence. If you dress like a bag of potatoes done up with string you will be perceived as one. Like it or not, you will be judged as first impressions count. Ipso facto !
Professionally speaking, it could make all the difference if you were hired or recommended. Your final photographic images are an important consideration and selling point, but it's the whole package that people pay for. People will choose a photographer for a variety of intangible reasons. You choose a restaurant for it's ambiance and not just for the food. Imagine a Chef who said my food is 1st class and award winning, but had a pair of dirty scruffy whites on, looked hungover and hadn't bothered dragging a comb through his / her hair - would you seriously be thinking I bet his / her food is yummy !
A good maxim is to dress for the occasion, if you blend in with the environment and wearing the same as everybody else you're probably fine. If you look around you and you're dressed completely different, you may want to reconsider your apparell.