Kudos to @actmyname to help the forum identify the worst spammer but whether it is negative, positive or neutral rating the guideline theymos gave regarding rating is very simple and clear.
- Do not rate people based on the quality of their posts.
That was posted before theymos changed the Trust system, and the feedback descriptions:
Read the description per Type carefully:

Note that the description for Positive, Neutral and Negative is different than it was 4 months ago: it was changed when Trust Flags were introduced.
My personal interpretation of Neutral feedback is this:
Neutral (shown as =1)
- Use Neutral feedback for anything that doesn't mean someone can or can't be trusted. This can be good feedback, for instance when someone helped you out.
- I think Neutral Feedback is currently undervalued on Bitcointalk. It's a great tool to de-escalate without drastic consequences. Please use it when appropriate.
And theymos said this:
LoyceV's guide seems reasonable.