If people do this, then later voters will have more influence over the post's final karma score than earlier voters. I used to think this was problematic because you're effectively nullifying others' votes based on your own judgement, but it seems more OK to me now for two reasons: * Earlier voters still change the visibility of the post even if they don't affect the final karma; the majority of viewers will see the post before it's approaching its final karma level. * Later voters may actually have more information than earlier voters, e.g. due to discussion in the comments or current events relevant to a post I do think of it as overruling someone else's vote, so to be cooperative I only do it when I feel like I'm much more informed than people who have already voted.

In the case of a pile on, wouldn't a comment be more effective than an upvote?