I think my approach is very similar to cog's and g0n's, and definitely asymetrical (thoughtful note by Pustular Postulant) but -- FWIW -- is expressed somewhat differently.

Voting is -- pre-eminently -- an opportunity to give something back to the community, which implies an obligation to cast votes only when one can add some (minimal) value; that is, when one believes that a vote offers a valid guideline to readers who come along later.

• ++ for nodes which will be useful (correct, clearly stated and provide appropriate examples or references) to the OP and others following the thread and which thus enhance the (already great) value of the Monastery.
  and, as this implies, rarely upvote questions unless they're related to issues on which I have or had a hard time groking the docs
• ++ for nodes which I find (a highly subjective judgement) inovative or intrinsicly intriguing.
• -- VERY rarely, unless I'm absolutely certain it's wrong (and at my skill level, being absolutely certain is very rare). Downvoting rudeness and nodes which are illegible or unintelligible seems (to me, YMMV) not worth much because that sort of behavior tends to provoke corrective responses (which are often even "polite") or to be the acts of visitors who'll soon "go away, anyway."