I agree, if the post was interesting enough to make you want to comment, it is fair to upvote the post.
If you dont have the vote slider thing yet, your upvote wont make a huge reward difference but in terms of morale of the creator it is worth it.
If you have enough Steempower to have the vote slider, then even just a 1% upvote would be fair, if you are leaving comments.

Another thing, if you are responding to a comment, why not upvote that comment. I see the trend of no votes happening a lot in comment trees too.

Not upvoting doesn't really help anyone, including comment makers.