This is pretty much always an informal guideline, and perhaps as a result the convention varies from board to board.
To what extent is Thread Necromancy a bad thing? As a novice mapper, I'll find myself searching for the same questions I'm asking so as to avoid repetition, but these threads don't always answer my question precisely, or may bring up wholly new questions. Is it okay to resurrect the thread by posting a reply? Is it unhelpful? Is it verboten?