When you see a question or answer you might normally down-vote that already has one or more down-votes, do you find yourself more or less likely to down-vote it? In other words, are pre-existing down-votes encouraging you to go ahead and pile-on or are they discouraging you?
- I choose which type of vote based on my opinion.
- I may choose not to exercise my vote if I feel that the existing vote count meets or exceeds the question's value (lack of value).
- I generally choose to try and improve a question that is poor, if possible, rather than downvote. I will downvote incorrect answers more readily.
- I tend to use close/flag for questions that don't belong rather than downvoting them.
I have no qualms about 0->-1 downvoting a question or answer that is poorly thought out or incorrect. If it already has a downvote, I normally won't add to it unless it's a really egregious example, or if it's a user I recognize as having a history of similar poor posts. In these cases, I will happily downvote them to oblivion just out of sheer frustration that they aren't learning. (These are the types of people who get posts on meta about them -- see posts tagged with [users] [behavior] for many such examples.)
I'll be honest, I tend to do all my upvoting/downvoting based on what score I personally think the post should have. So if it should be -2, and it's at -1, yeah I'll pile on. If it's at 24, and I think it should be 20, I won't downvote (because it's obviously good!) but I won't upvote, either.
So no, I don't pile on. I just vote based on where I think something should be.
Of course, the rules are totally different on Meta. On Meta I vote based on agreement -- up if I really agree (or you saved me the trouble of writing my own), or down if I disagree. (In fact, I have more Meta downvotes than all other Trilogy and StackExchane downvotes combined for my OpenID.)
Most of the time when you see something hit -3 or less it should simply be flagged or closed (although sometimes that Grammar Gestapo have just gone overboard on someone for whom English isn't their first language) so there doesn't seem much point on downvoting. This is particularly true for questions that simply need to be migrated.
I factor in current score before I down-vote. I rarely down-vote questions unless the person asking the question changes the meaning of the question in the discussion - then I down-vote for the obvious hideously worded question.
I will up-vote just about anything I agree with - especially those with low scores.
I will up-vote an answer, even if I don't agree with it, if I feel it was unfairly down-voted to bring them back to "0".
I will down-vote any question that is flat-out wrong (but I won't pile on).
Finally I will down-vote any question that questions the motivation of the original poster.
I completely avoid pile-on downvoting. It just seems really snippy, and I can't imagine that anyone - from the answerer to the community - gleaning any useful information from a -8 vs -9 score.
I'm much more likely to pile on an upvote, probably because I am more of the "reward good behavior" rather than "punish bad behavior" mindset.
I kinda disagree with the idea of downvoting because of "frustration". Its like saying "how dare you waste the time I spent on the intartubez by your useless drivel!" which might be someone's sentiment, but really shallow. Pile-on downvoting is like the SO equivalent of a flamewar. "j00 n00b, ur stoopid gtfo"
Serial offenders (if you can even call "posting on the internet" an "offense") will be dealt with by the moderation team anyway.