Took a quick look at the tag -ass and realized there's 200 pages. Of the first 10 pages, I see ass applying as a tag to every image, and that makes sense given that it would be quite hard to have a composition where there is a butt crack but no ass.
My concern is that this kind of implication / tagging policy would turn ass into another breasts, applied to anyone who seems to have an ass, regardless of whether the body part in question is the least bit prominent in the picture.