Generally speaking, you want to ask the OP if it is OK to post in their thread. If it's clear that the person hasn't been around for quite awhile, then it's probably OK to post there. If they have been posting elsewhere, it's best to check with them first.
Also, a lot of stuff gets deleted or sent to the archives because it recently had already posted. I have a bunch of photo sets I was getting ready to post but someone else started up a thread with those very sets, so I've waited and only posted one every once in awhile.
I know in the video sections it's all but impossible to keep track of what's been posted. I honestly don't know an easy way to keep track of it. Which is one of the reasons I didn't want to mod there!
I hope that helped.
JB