When I was younger, I kept my pistols in cases with the foam egg crate material. I never saw any ill effects. - However I also lightly sprayed a piece of cotton t-shirt with a spray oil, wrapped that around the pistol, and then put it in the case. I now have a gun safe that holds everything, so I don't use the cases for storage anymore. -But I think the key is to not just close them up and forget about them for long periods of time. That goes for any type of storage. If any moisture makes its way to the firearm - whether that be from humidity in the air, a water leak, any other source, its going to cause an issue. If they are not being checked on regularly, that's when rust gets a chance to develop.