[quote name='MoCiWe']When I went to see Scott Pilgrim on launch after the movie when me and my buddy were walking out of the screening an employee handed both of us a poster for the movie, so it seems like most people are saying they probably judt try to hand them out if nobody that works there wants it. This was at a Marcus Theatre btw.[/QUOTE]
Almost certainly a promotional item, not a true one-sheet.
Anyway, I have experience working at movie theaters, and it is as people have already said. It usually depends on the theater, but almost certainly the best stuff is claimed by the promotions manager or the general manager, then other management, then employees.
I can verify that there are cases where distributors want the posters back, but that only seems to apply for super narrow releases, where the companies want to save all the money they can. And posters are destroyed usually when there is an error or correction, like the Shutter Island posters with the October 2009 release dates on them, or concerns over content (One of the Saw movies had this problem, if I recall it was Saw 3.)
And just for any aspiring collectors, nearly every legitimate movie poster is double sided, with a mirror print on the back, for display in lightboxes. That being said, there are exceptions in both ways, with some official posters being one-sided, and some re-prints and knockoffs being double-sided.
/shameless bragging: My best items are a Batman Begins poster, Hot Fuzz poster, Incredibles poster, three Transformers posters (The Prime and Megatron faceoff set and the theatrical), Harry Potter 5 teaser (Voldemort) and theatrical, and a Sin City 7ft vinyl banner with Jessica Alba on it. There might be more particularly good pieces, but these are the ones I remember right away.