It's one of those controversial topics on what's more professional almost like military vs. civilian pilot candidate. With the obvious answer though.

AA, UAL, DAL require them. CAL only when wearing the jacket. Correct me if I am wrong.

What I noticed is that when people stop wearing hats too many of them overtime stop shining their shoes and even ironing their shirts and taking their pants and jackets to dry cleaning.

Is it connected to the profession going down the toilet? I think it is.
It is much easier to convince people that you are underplayed and overworked if you stay professional.