So...
Thank you posts or not?
Sometimes?
Case by case?
It depends...take the J-Urban forum on JPM (FOR EXAMPLE), a lot of people there get whiney - and rightfully so, when they put up a YSI link, and it goes down by the third response. With problems like that, the least the uploader wants you to do is thank them for taking the time to upload it and bring it to the board.
Another EXAMPLE, is the MM audio/video forums on JPM, where the ONLY posts some people seem to make are "thank you" posts. Aren't there a few guys with over 100 posts of NOTHING but "thank you", "thanks", "ty", etc.? That becomes annoying because you have these idiots who don't even contribute a thought, a download, or atleast three full sentences. People are justified in being annoyed with that case.
So basically, on a forum, the "thank you" post (IMO) shows that you're atleast a greatful leecher. Add an opinion with your "thank you" post, and you'll probably be respected even more because you're atleast contributing to the conversation. What people these days do not realize is that forums are meant for
discussion. People uploading for the forum is a priveledge, but since that has become such a norm in the "Internet Jpop Community", people have forgotten to actually talk - or atleast thank the initial contributor.