http://filmschoolrejects.com/features/movie-poster-storage-ideas.php
backing boards are neutralized. Not for forever, but for years.
will they help posters flatten over time if stored as sandwiches? sure they will.
I don't use boards and I don't bag my posters (it does not make them easier to handle, quite the opposite) what I do is lay a poster on foamcore in my flatfile, and then put in a plastic bag, another poster, plastic bag.
the weight of the bag is better than any other medium and it caresses the posters in all spots, unlike a stiff cardboard
it's worked for me for decades
I don't bag my posters. I don't bag & back them.
as to the link above.. I think it's one of many good ideas