I've been noticing an interesting trend since the "thumbs up, thumbs down" option had been installed in the forums. And that is how someone would retaliate against a person who had given them a negative rating by finding one of their posts and doing likewise.
Here's my take on the reputation options: it can be a valuable tool. When used right, it will give the poster a sense (for the most part) of taking a mature responsibility to their posting as no one wants a ugly negative under their name. This also helps the moderators immensely because people "should" be not as quick to post something offense lest they bare that negative number.
However, I have seen it abused. I've had it done to me twice.
When I give someone a positive score it is always because their posts were well thought out, intriguing, thought provoking or conveyed a general sense of wonder.
When I give someone a negative score it is always because they publicly humiliate or put down another poster or some other immature action that is usually against the rules of conduct. I never give out a negative score simply because I disagree. It's always when they personally attack another.
Now, with the rating issue, I have noticed with me is this:
Whenever I give out a negative rating it is always warranted and the proof lies in a moderator responding shortly thereafter to the offending poster to "stop it." When I give out these negative ratings, each and every single time, the offending poster will seek out on of my posts and just give it a thumbs down without any rhyme or reason.
Now, this can be problematic for the moderators as they go into my rating and take the negative rating off. This isn't cool because it is more work for them over such a childish act of aggression.
So my opinion is this: Do not blindly give out positive ratings because someone gave you a positive rating (professional courtesy) and do not blindly give out a negative rating because that person gave you a negative rating.
If you feel that the rating you were given was unjustified, do not be immature and attack back. Approach a moderator in private and let them know and have them review the case for themselves. For those who got a negative rating who really deserved it, do not act like a child and give that person a negative rating because they made you mad.
If you rightfully deserved a negative rating, let it be a learning experience for you and realize that if your rating is like -5 and your positive is like 1, you may have a repeated pattern of behavior you should reflect upon.