I tend to upvote a post when it Links an interesting article or provokes an interesting conversation, and only downvote them when they voice an convectively bad opinion (like, bigotry or other unjustified vitriol).
I tend to upvote comments when they make a good point or at least try to support their argument, and only downvote them for the same reason as posts: when they are trolling, being unnecessarily aggressive, or trying to justify bigotry with fallacious reasoning or something
What about you?
I try not to use downvotes unless someone expresses a truly gross point of view or outright says something that is wrong / counterfactual. I upvote the ones that stand out as useful / informative / funny / insightful for comments. I upvote most posts if I enjoy seeing them and any that are requests for information.