I always I++ the node if I learn something. I usually I++ something if it is a good, well thought out post. The only time I really I-- something is if its a negative post or if someone obviously just wants an awnser to their problem and has done no footwork themselves. Im sure I too have other reasons/rules, but the latter are the basics for me.

Cheers
Ted

UPDATE

Id be curious to know how many people will I++ a reply to a node of their own if the comment/reply left is a blow to their ego. How many would I-- ? My rule for this generally is that I still I++ if I have learned something. Id only I-- a post of this nature if it was overtly negative. I really try keeping my own bruised ego in check.