For someone who is intending on getting their FI rating in the next few months, the topic of attire is one that I have thought about and I don't have any problems with wearing a tie. IMHO I think any instructor should be wearing a collar and tie. I would intend to pitch up to meet my student looking smart with a collar and tie and shoes as polished as they would be for a interview or flight test. Your student (don't forget customer!) is far more important than the CAAFU chap who sat beside you on your IRT, because the student is paying for the privilege and is ultimately paying you! So if you made an effort to look tidy for your IRT then you should be making even more of an effort for your PPL student.
First impressions are of vital importance, especially for anyone who is looking to part with cash and large amounts of it too! Smart instructors, tidy clubhouse and a clean fleet of aircraft will create the right impression for the average punter who wanders in on his lunch break to find out about getting a PPL or even just an hours gift voucher for his nearest and dearest. To cut it short I feel that a commercial pilot is regarded as a professional and not many highly skilled professionals turn out in a T-Shirt and Jeans for work.
Call me 'old school' if you want, but I have heard customers who say that the very least they expect is a well turned out instructor - I can't agree with them more!
Happy instructing folks
MG