I think for original content I'll always upvote myself on the basis that if I didn't think it was good content I wouldn't have posted it....

For comments however I think the jury's out a little more on whether my contribution is good - it's more dependent on whether it promotes further discourse or adds to the conversation - obviously I think it is but I think the mechanism of voting allows us to curate that as a community... and it's a lot harder to be a bad community member in an original post without outright trolling.