Yes, it's about respect and professionalism. I doubt anyone would have said anything if I wore a polo to my 74 oral and type, but the check airman was in a tie, the fed was in a tie, and I'd have likely felt underdressed otherwise.
They don't tell you to shower or brush your teeth before ground school, but most of us do anyway. YMMV
By no means am I judging the 76 guys who train in a more relaxed atmosphere. When in Rome.