Sometimes though when things are really interesting but long I'll go back, upvote and then continue reading to prevent it from dropping off the page.
This is especially true if it is a story from /newest
Also with a few web sites I might upvote it first and then read it :-|
I can neither confirm nor deny that I do this too but I can say that I think it makes sense to do this, and I would go as far as to say that I think it's healthy for the HN community that stories get upvoted by people who have not yet read the story in full, as long as it doesn't happen too much -- while a story might turn out to be lacking or even factually incorrect in some way I think there is opportunity for interesting discussion to be had as long as the topic at hand is interesting.