I try to not down vote for disagreeing, because most of my opinions are wrong. I have changed almost all my positions more than once and it will happen again. If people disagree without trolling it is valuable, even if they are more wrong than me. It helps me be less wrong.

I have started to down vote when people wastes my time: Trolls and sometimes for boring flame wars that show up in every discussion on a subject (like Perl 5 trolls that obviously don't know the language, but has Googled a reference to the operators of Perl 6!).