I regulary participate in 10 of the FSM forums. When I post, I ordinarily post to what I think is the most relevent forum. However, for questions that require an answer, and you think there are a couple of forums that might be relevent, I see nothing wrong in multiple posts.
I have seen trolls who post the same rant in a dozen or so forums. Don't to that. But sure, for a legitimate question, go ahead and post to a couple of forums.
I might also suggest taking a look at the forums in the Tools, Techniques and References section. This gem of a group is easily overlooked, because it is down below the Modeling Subjects section. But it has invaluable threads on tools, paints and painting, and decaling. Not a lot of traffic on it, but it would be even better if there were more traffic there.