It amazes me how much a single downvote bothers people, yet how little attention they seem to pay to the cumulative measure given by %positive. You may be interested in Phil Goetz's take.

Your 30 day karma is at 96%, but your overall score is at 89%. Do you see a difference? Added: another way of saying it is 11% downvotes vs 4%: twice as many!

Note that %positive score is the percent of votes, not, say, percent of comments that end up positive. I think you should give more thought to what you should be optimizing for.