I don't think they do! Blue and Piper do this on the back of my desk chair too. Sometimes I forget they're there until I stand up, and the chair reclines back with a "thunk". They have yet to fall off though, so I believe this balancing-on-a-tightwire trick is an innate feline skill.
Poor Molly with her eye troubles. I hope it gets better soon!
my siamese is like that too! she actually jumps to the top of doors and will lay there until we take her down. we don't like that she does this, we are worried about her hurting herself! she is 11 now! one time she jumped up to the bar in our closet, walked to one end and TURNED AROUND and came back, all on that tiny bar! gravity does not have an effect on her, only on the things that she knocks down as she walks along the edge of things!