i think it would be weird to thank people for upvoting without a comment - would you tag them?
Also, I don't think you need feel obligated to thank on one of their posts - but leaving a comment is always appreciated, just make sure it is about the post you are commenting on :)
That is my dillema, to thank them for the up vote seems strange. But I feel obligated to repay the debt?
So reply to a comment, up vote some of their content in return for up votes given. Do You feel this is more the way to go?