I only really use the Thanks button on other people's threads as way of saying "Thanks for posting that, you've said exactly what I was thinking/put what I meant so much better/I haven't time to respond but this is exactly how I feel/Thanks for setting somebody else straight" kind of thing. I don't tend to start many threads of my own but if I do, I would normally just say thanks to everybody in my post when I reply. If a couple of people have replied but I'm waiting to see if anybody else does before I reply, I will sometimes click thanks just to acknowledge the replies already there, to show I've not ignored them. I wouldn't ever go down a thread with more than a couple of responses and thank them all, that just seems to take away the meaning of it (which to me is to thank somebody for a particularly good post, not just general thanks for replying).
But then to be honest, I never notice if somebody has thanked me for a post I've written. I just read new replies, I never go back to check my own posts in a thread.