I upvote all comments I get usually, some at a lower % than others, but everyone gets a little something because if they commented, they probably read my blog and have taken even a second out of their day to say something nice, even if it is a short sentence or compliment. And that means a lot to me, so I upvote them. My one exception is usually the people who say something like "upvote me and follow me," haha of course that is just not worthy.