Have come to this topic late so forgive me if I go over old ground.
I was a full-time instructor for 2 yrs or so and now do the odd bit of ad-hoc instr - mostly TLs. I believe that the instructor should wear a collar and tie and be smarlty attired - it's called looking and being professional for goodness sake!! I personally would not feel right turning up to work in anything other than black and whites - and no, I don't (and never have) worn crunchy bars in my capacity as an instr. As a paying customer I would expect my instructor to be dressed appropriately. Call me old fashioned but to me, the poll results and the existence of this thread are indicative of the times we live in.
Prepare to be shot down by the PC brigade!!!!