To add to what others are saying, I agree about trying to post correct rules information.
I understand not everyone knows fiddly parts of MtG's rules, and most people aren't judges, but I would make it clear in a post if I were unsure about a ruling and try to link relevant info. I also understand downvoting factually incorrect posts or upvoting correct ones. Some people care more about their karma than others, but at the end of the day, karma's just internet points. Having a higher number is nice, but all it earns is the higher number.
I think making clear exactly how confident you are in an answer to a rules question would be a good step to avoiding downvotes. If you're not sure, just start with "I'm not certain" or "I'm not a judge, but..." I think the negative reactions are to the appearance of certainty when the answer's just a guess, since it can be misleading for new players.
I don't think anyone should be afraid to post a guess to a rules question, but I would like them to state that it's a guess and what they're basing it on. If someone's pretty certain they know the right answer, it would still be nice to know why they're certain.
EDIT: Are there forum rules somewhere about what exactly constitutes appropriate times to upvote and downvote? I'm not sure we really need that, or even how it could be enforced, but at least codifying it would give members a clear understanding of what might lose them karma. As said above, I would be in favor of downvoting inaccurate posts (and removing the downvote once they're corrected). I don't currently do this, but it might give a clear system for alerting newer players to which answer is considered correct by the community.