As a complete aside...but on the subject of what not to wear...
I was in LAX the other week and passed a number of US flight crew wearing Airline issue leather bomber jackets!!
Now is THAT pretentious or not!!..but hey!, that's America for you!
On the subject of shirts and ties, my vote is yes. Instructors may not be paid as a professional, but they should at least look like one. But PLEASE, ...no stripes or wings

unless...the pay is good enough!!