Wikipedia has this to say, but this is more applicable to commenting than voting:

Bugzilla is a place to notify developers of bugs and contribute information that would be useful in the fixing of the bugs. It is not a place for advocacy viz. "I REALLY WANT THIS BUG FIXED!"

More directly, bugzilla has an etiquette page, but it doesn't cover voting.

Personally, I'm of the opinion that you should be doing everything you can to help the developers find, prioritize and fix bugs— voting is an effective way to help them do just that (well, when people actually vote on what's important to them :\). This isn't like voting for yourself in a political election where one might submit that you have a conflict of interests, you're indicating to the developers what's important to the community and you're a part of the community.

I vote on the bugs which are either important to me or I believe should have a high priority (like the 24h bug which, while not important to me personally, I believe is an important bug), and, usually, bugs that I submit are important to me so I end up voting on them. That said, I don't vote on all of my bugs.

In summary, vote on what matters to you.