I comment just the contents that I upvote, even if sometimes I upvote without reply to the post. The only contents that I comment without an upvote are content of plagiarists (I usually flag the plagiarists). I think there are some people who comment without upvote because: 1) they want your vote on their contents and 2) they want just upvote their own comments.
I really can't understand a 0% upvote, I think people give a 0% upvote just to say you "hi there, I upvote you so you have to upvote me". Really bad. I usually give my upvote between 15 and 25 % , so I can give some upvotes around.