I have seen many PPL instructors and know many as well. I have never met one who has not worn a tie when ever he/she turns up to work. Therefore after years of seeing this, it has become the norm for me to see a tie than not see one. This includes people who are wanting to join the airlines and those who do it during the weekends for fun. Even the chaps that I know who fly for the airlines still wear a shirt and tie on their days off to instruct! However personally speaking I would still wear a collar and tie even if I was the only one doing so!
None of us would probably make any fuss if we are required to wear ties in an airline environment and to be very honest with you I don't see how PPL instructing should differ in the slightest!
MG