It's a good policy to not comment on a woman's appearance unless you have already established that type of rapport in a relationship, because 1) without context it reduces her to less than a whole person and 2) it assumes she wants that form of attention from a stranger, which is a big assumption to make. It's not about your intent, it's about how the comment is received. Complimenting a woman on her hair/boots, etc, is slightly more acceptable, because you're not making an unsolicited judgment about something inherent to her person.
My thoughts.