Obviously, upvoting your own question as an alt account is voting fraud, but what if I were to downvote my own question as a sockpuppet? How would that look?
The rules state you're not allowed to do anything with multiple accounts that you couldn't do with a single account. Voting on your own posts is something that you can't do without the additional account, so it's violating the rules. Maybe no one would care enough to report you or to act on it, or maybe they would, but it's a violation of the rules nonetheless.