First off let's be real: There's almost nobody out there who doesn't know you're supposed to shave and wear biz casual for the jumpseat, everybody gets told that at some point during indoc. Ex-mil guys don't wing stuff like that, they'll look it up or ask their buds. Regional noobs might possibly have not been told, since the commuters are in such disarray these days.
Old guys should really have no excuse because you used to need a jacket and tie back in the day.
My personal philosophy is a reasonable intersection of written policy and the current enforcement climate. If memos are flying or people are getting carpet dances then I'm going to deny someone who's grossly outside policy. Otherwise I'll just politely mention the policy for future reference to hopefully save some hassle for someone down the line.
It also matters if the flight's originating at the mothership during bankers hours when all of the Company Men are out and about.
I'm not willing to get on the radar or do a carpet dance on my day off because somebody is feeling a bit entitled to something that's more of a privilege.
I jump in jeans all the time but they're khaki colored and pass for biz casual. I also wear a collar of some sort... I own a couple collared sports shirts for graduations, jumpseating, etc. All of which is comfortable for laid-back daily wear.