Long time lurker, first time poster.
I'm just popping out hiding to say that I've never particularly seen anything wrong with this either and have done it on occasion myself (though I always make sure my feet don't touch any of the fabric of the seat). Ushers have never spoken a word about it to any one and as long as you're not blocking a seat that's needed (i.e. wait until the movie's started to get settled), I don't see it as particularly rude; it's not...delicate
but not rude.
Saying that though, I would be highly uncomfortable if someone's feet were dangling right next to my face while I were watching a movie and definitely think that you're well within your rights to say something about that. Or if they're blocking a seat you want to sit in. A simple, "I'm sorry but I'm not comfortable with your feet there, could you please move them" or "I'd like to sit there, would you please move your feet" wouldn't be out of line. Saying it to someone you're not even near would come across as a bit condscending.
Alternatively, if it really gets under your skin, just duck out and go ask the usher about it. They'll either act or not depending on what they deem is acceptable.
Honestly, I'm more squicked out about possible lice on the headrest than whether anyone's shoes have been in the top edge. It's a weird fear, not probable but possible. (This after having a horrific episode when oldest was in kindergarten - *shudder*)
^ Also this. If I have a jacket with me I sit with my hood up. It makes me a little more comfortable.