If I round up, it makes the game look better than it is. If I round down, I am being giving an inaccurate portray of how I really feel.
You are but one drop of rain in a monsoon. Ratings *should* appropriately dither over the aggregate, so the few who are in the middle will likely half vote up, while half vote down.
While up/down may not be entirely fair, there's really more options for manipulation in a star rating system. By removing zero-star ratings as an option, they can artificially inflate scores. By changing the textual labels (what if 4-stars was described as "Just Okay"?) they can manipulate people into rating higher. And in general, companies are biased to WANT higher ratings, so you'll be more likely to stay around longer, spending more money, so whatever system they design is going to err or the high side.