Stripes when instructing are not necessary. Maybe allowances can be made for commercial flying but certainly not for PPL instructing. At the end of the day PPL flying is all about flying in a relaxed atmosphere and having fun. You want to be approachable to your students and not create a 'them and us' culture with you with your 4 silver or gold bars on! My CAAFU IR examiner did not wear any stripes so if he if not going to then I as a PPL instructor will definitely not even consider it. You can still create a professional impression to a customer at a PPL FTO by wearing a white shirt with dark trousers and a dark tie and polished shoes!