I tend to up-vote a good deal of comments, but -- like you -- I do it more the first time I read a post's comments. So, any comments written before I read it are more likely to get upvotes than any that are written later and which I see on later visits. (This is compounded by the fact that I'm trying to cut down my lurking on HN, since it's so easy to keep reading all the good stuff that other people write.)