My personal voting rules:

• First, I try to avoid giving the same user two votes in the same thread. Especially when they are just saying the same thing a second time.
• Second, if I think that the post is somehow un-professional, I give it a --.
• Third, if I strongly diagree with what is being said, I give it a null vote. (Sometimes, I wish there was a "I vote not to vote on this" option, so I could see the reputation of a node without having to modify it up or down).
• Fourth, if the node has no meaningful content, I consider it.
• Finally, if I actually take the time to read and think about what was posted, I typically give it a ++.

Some people might disagree with that final point, but my philosophy is that any meaningful contribution is worth something, even if its completely wrong or if it appears to be a rediculously simple question / answer.