I save my down-votes for replies that are rude, nasty or condescending; especially those directed at a poster who is obviously new to Perl, Perl Monks or both.
In particular, I find that there are a few monks who feel it is necessary to berate people and opinions they disagree with. I try to discourage this with down-votes, but I think the only effect of this is making me feel better.
I'd like the monastery to be a bastion of tolerance and civility in an internet that is largely lacking in both.