When you ask for advice on a forum, and then someone replies, is it customary to thank your respondent afterwards? Or is this considered an unnecessary waste of forum space?

I don't know if there's a hard and fast rule one way or another, but on all forums I've participated in, I always come back and say thank you, comment on the post and/or rate it. I appreciate when someone thanks me when I've answered a question, too. That way I know the question helped, and also that the person actually read it!

To me it doesn't matter, but it is always good to see someone reply and thank the responder. It means they read and undestood the point.

Hi, Anna,
You have a very good question.
As another stated, there are no "rules" for this.
If you wish to show your appreciation, you can click on the "Comments on this Post" link, add your comments with "approval" or "disapproval" checked.
But, you don't have to.
One can always send a Private Message showing thanks, but not have to.
One can also Post a "thank you", which is good because it lets us know that everything is OK, and if our answers have helped in any way. But, you don't have to.
I do wish you the best, and have a great day.

It is never a waste of time to say thanks. Posting back with what works and doesn't work helps make the site work and means more to me. There are some very good people here with outstanding knowledge in certain areas.

Actually, comments can be used to express thanks and to let us know if the post was helpful or not.

Well, thanks. :) You've all cleared this up for me I think.
From now on I will just re-post to show thanks and let people know I saw their responses, without worrying that it's silly.

It's always a nice gesture. Of course, sometimes certain posts receive many replies so trying to individually thank every one can become a bit cumbersome. An alternative can be to simply post a single "thank you to all" once you've received all the replies that you feel you need or want.

I normally don't go back and check unless I see a thread coming back several times and then merely to see what a few others have written.

Thank you are nice, as is feed back but normally most of us just do this because we like to help.