Usually crotch, unless I'm feeling self-conscious, then it's ass. Sometimes I just want to avoid eye contact with people when I'm grumpy or something to that effect, so I turn away from people. When Im in a good mood, I turn to face people and smile.
In case I accidentally trip and fall on someone, I think it's best to fall forward and hug someone than fall backwards and sit on someone!
So basically I wouldn't think of it as a ass/crotch deal, but an eye contact or no risk of eye contact sort of thing. Sometimes looking at people straight in the eye is too much for me. I tend to be on the shy side.