Hey Mike..
what is the poster you are maybe wanting to back? Are you wanting to back it because it has damage, tears, rips, that kind of thing? There are a couple schools of thought--but it ultimately boils down to a personal decision. Many feel that to back a poster just to remove fold lines is, in a sense, "overkill" and should only be done when posters are fragile, damaged, missing paper and the like.
Others do back because it smooths out fold lines and creases and they can be easier to frame and handle, many times.
Is your poster in question a folded or rolled one?
Do research and take time when selecting a restorer. You dont want someone who is going to do more harm than good. All restorers should use water based paints, colored pencils and adhesives (some don't).
Jeff