I have seen many post over the years discussing the negative effects of the Ultra-Pro 9-pocket pages on the thinner, tan-backed Wacky Package stickers. If left in the pages as-is, the stickers will usually take the shape of the "waves" in the plastic sheeting. While time consuming (any money consuming!), I've found a way to prevent the inherent warping, and also prevent vertically stored binders from bending the bottoms of the 9-pocket pages.
For the separating inserts, I use black, acid free, heavy gauge matte board that can readily be found at stores like Michael's, as well as many art supply stores. The vertical measurement of these dividers is approximately an 1/8" taller than the top & bottom of the 9-pocket pages. The bottom 1/8" supplies a buffer between the pages and the shelf.
I took some pictures of a couple of the filled 9-pocket pages in oblique lighting to illustrate how flat these dividers keep the pages.


