Some local game stores (e.g. Funcoland) selling used copies of NES games included the games in
cardboard sleeves (almost certainly recycled cardboard). I have lots of these laying around. I'm sure whoever thought of that thought they were being clever, but here's where their idea failed:
After a few years, cardboard sleeves being exposed to oxygen and general environment effects (esp. humidity), the cardboard begins to break down / flake off. The flakes tend to go right into the NES cartridge slot. It's just paper, so in most cases simply blowing out the paper specks works fine. But in more volatile situations, the bits end up sticking to the edge contacts and don't come off by blowing, requiring you clean the edge connectors with proper cleaning solution.
Using plastic is a generally better idea since it isn't as susceptible to environmental breakdown. I still have many of those
classic polypropylene NES sleeves (including the non-branded ones) and they're still as good as they were back then -- just with dust at the bottom of them. There are some official and third-party
plastic containers that actually fit the entire cartridge (I have a few left from my childhood, as does
Pat the NES Punk), which IMO are probably a better choice all around.
Reminds me of a line from the Beck song called Cyanide Breath Mint: "I got a funny feeling they've got plastic in the afterlife".
Anyway my point is that paper generally doesn't hold up over time. Find a different substance if possible.