I have no problem with cross-posting all at once, until you're spamming, say, 10+ communities at the same time. (I've mostly seen this on various spirituality-related comms, but I'm sure it happens elsewhere, too.) If you feel the need to cross-post that many times, 1) at least have the decency to cut, and 2) if it's a poll/survey, I'd almost prefer you spread it out over a couple of days -- it'll probably be easier to deal with the data that way, anyway. IMHO, it shows you're actually interested in the results, not just spamming to see how many responses you can get.