Not usually with mirrors, but with other reflective surfaces, all the time. Windows, shiny buildings, etc. I really love my winter coat, it’s super cute, kind of stands out (bright, leprechaun green), and gathers over the butt a bit like a riding jacket. So whenever I pass a reflective surface, I’ll often check out my butt. There. I said it.
If I’m not wearing that jacket, I don’t generally bother unless I see something very wrong out of the corner of my eye (like my hair pulling a Pippi Longstockings or something).