Couldn't agree more with harrip. I believe that if you demonstrate a sound degree of general knowledge and an ability to handle different situations you will pass. However, when I was an instructor and a student turned up for a lesson that was wearing an outfit that would put Ali-G to shame I couldn't help but wonder if this guy was really serious about his flying. I tended to help and offer more advice to the ones that demonstrated their seriousness about a career in aviation.
Lets face it peoples, if you want to get anywhere in aviation you need to network! Testing officers normally have a wealth of experience under their belts. They've done the rounds and know a lot of people in the industry. If you can prove to them that you’re a serious 'package' (not just a good pilot) it may just help you with a reference in the future. It’s the little things that help us move forward and out of GA.
If you are taking the instructor path after you obtain your CPL, I dare say you will be wearing a white uniform and blue pants, so why not buy it now (forget the epaulettes though). How much have you spent so far and how much is a shirt and pants? Anyways good luck