I must admit I work (as a ground instructor, not yet as FI) at a school that also does a lot of commercial training, but certainly I think it best we wear ties. However the people working for the little PPL-only club could probably get away with a club polo shirt and slacks I think.
Note that many (not all, we do not insist) of our commercial students wear a simple uniform both in groundschool and on the flying side. It does start them in a professional feeling, plus to their own self-esteem it distinguishes them as career pilots from the hobby pilots without seeming too pretentious.