As very frequent flyer I'll use points to upgrade if there's availability - often my flights are at short notice so I'll ask at check-in. There've been a couple of occasions I've been asked by the Crew if "I'd mind moving up forward?", and I don't mind at all - I like to think it's recognition of the "get what you give" attitude.
Obnoxious, anything "challenged" SLF don't deserve the time of day! More often than not they've deliberately tried the bluff/bluster so give it back to them "in the nicest possible way" of course.
If you didn't pay for it, it's not yours to take! If the crew offer, be grateful, but to take advantage of busy crew is deceitful -
!