A quick bit of motivation: recently a question I answered quite a while ago ( Combining Two 3D Rotations ) picked up another (IMHO rather poor) answer. While it was downvoted by someone else and I strongly concur with their opinion, I haven't downvoted it myself because I'm leery of any perception of 'competitive' downvoting on questions that I've already answered; in general I tend to be very stingy with downvotes (certainly more than I probably should), but this seems like a particularly thorny case.
What I'm wondering is whether this is a reasonable concern (or reasonable approach) on my part; do people concur that this is something to be worried about from an ethical perspective, or should a bad answer be downvoted regardless of whether it might be abstractly 'beneficial' to myself to do so?