You don't do that because you aren't sure what scale the asker has in mind when they ask you to rate yourself on a 1-10 scale. If Paul Graham asks me that ?, his view of a 10 is going to be significantly different from some recruiter whose last job was managing a Dairy Queen.

I've handled this by giving my own rating as well as who I think a 10 would be. For instance, I'd say that I'm a 5 but my 10 is someone like Bill Joy or Linus Torvalds - truly world class developers.