You should report it anyway so a moderator can move the thread to the proper location.
Just don't post in the topic saying that it's in the wrong forum. That's mini-modding, and moderators hate that. If it's a question that needs answering, feel free to answer the question. Just don't bring up the thread's location in your post.
No, it's not. Some people think they are being helpful, but really all it does is annoy the moderators. Let them do their jobs - everyone else's is to use that report button.