We could spark a second thread about formal dress on FI's really couldn't we? Personally, I dislike wearing a tie for my job but understand they have their place in other pilot occupations such as airline or corporate pilot. A tie on a busy flying day restricts my comfort and creates an impression that is above and beyond the level of formality I seek to create between myself and the student. So I don't wear one. My boss is happy with that and it fits in with his own ethos. The other instructors are free to wear a tie if they so wish - that too is their decision.
VFE.