I think it depends on the thickness of the cheeks and thong/string. I have a thicker than average booty for a guy, and I usually just feel thongs rest between my cheeks. If I start doing some crazy stretches or yoga I'm sure it'd get a little closer to the anus as things spread out.

Strings definitely get more up close and personal, but I feel it more on the top of my ass where it meets my back and on my gooch than my anus directly. It definitely goes a little deeper between the cheeks though, but I sure don't feel it rubbing against my anus. The right thong/string should feel like almost wearing nothing.