I too have always wondered whether to thank or not. While I was always taught to thank someone as a youngster who has helped me, I do see that it can well clutter the thread. In the past I have thanked both with private messages and posts.
Reading the entire message string it is sometimes difficult to see if the posted problem has been answered to the satisfaction of the original poster.
Recently looking at another help forum (PHP) which asked that all those that had found a suitable solution to their problem should raise a flag which indicated problem Solved
This to my mind gave some positive feedback to the member who posted a suitable answer but also stopped others posting unnecessarily. That may or may not be a good thing here as many subsequent answers to posts also have great merit.