I think it's not logical to vote on the past (well, actually, yes, but not on the past that has changed). When you rate per user, you take into account everything you remember about that user, if he WAS helpful in the past, or maybe if he WAS a son of a bi*** and he realised he was wrong and changed. With per post rating, after a couple of weeks/months, everyone's reputation will be freezed because to change it hundreds of ratings will be needed.
Rate on present. After all, reputation matters for now, not for the past (well... except at 88mph in a Delorean
)
Reputation: users or posts?
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