Can't say what other people do, but I don't rate my own games 'cause that would distort the average. Besides, what's the point? To make me feel proud of my own work? Already am.
That said, when I rate someone else's game it's based on how much I like it. Not genre, not how it compares to others. If I just don't care for a game, I won't rate it at all rather than punishing the creator for having different tastes. It's only fair.