It's never bothered me unless someone's feet were on the seat next to me, which has happened before and I recall the guilty party taking her feet down after I turned and looked at her (I was a bit incredulous that she thought I'd be okay with someone's feet right next to my face).
I'm also pretty short, and I will often have to prop up my feet because they, otherwise, will dangle (it took me YEARS to figure out why I hated certain seats so much, like armless chairs, because if my feet were dangling, I needed, at least, to have something I could lean against so I could take the strain off my dangling legs, sometimes tucking a leg under me, which wasn't quite so easy with armless chairs).
I also figure if the theatre thought it worth policing, they would tell patrons not to put their feet up (we certainly are reminded all the time not to talk and not to use our cell phones). The person who put up the biggest stink about where my feet went was a friend. Even if I had no shoes on but socks or bare feet and it was just the bottoms of my feet touching the back of the seat or, even, my foot, shoeless or not, against the back of the seat because my legs were crossed (not hanging over the backrest), he'd still make a comment. I think I learned to ignore him.