I don't know, there's enough people around to do the negative bitching thing, so I don't worry about it. However, I always give positive feedback where warranted. For example, picked up a few themes on ThemeForest just recently, one of which I really liked, so they got a nice note from me in their comments section, which will hopefully bring them a few extra sales.
I dont even pay attention to positives and reviews from certain people. I noticed a lot of sellers giving each other great reviews. I might be burning bridges here; but its the truth.