No rules really, just if you like the post the you can give it some reputation points.
I think that is were the confusion comes in is that some members see the reputation system as a system of trust like on bitcointalk but it actually has nothing to do with trust.
Our system is there to up or down vote posts on the forum.
I think it needs a name change, it would be much simpler and easier to understand if it was a simple up/down vote.
I will add it to my to do list if others think the current name of the post voting system is confusing.
I'm guessing you're going for a sort of stackoverflow kind of system? Theirs is working out really, really well. If people become more active when upvoting/downvoting, you could do an option to list posts by vote.