I'm probably going to start another storm of controversy over this but I'm going ahead anyway.
I have two types of cases, a 200pc wallet labelled "Keeper" and 32pc Case Logic case. Now then, The sleeves in the "Keeper" wallets have some kind of backing material which is white in colour. The Case Logic one has backing material which is black. Both wallets are stored standing up vertically on their sides, so that the discs are not pressing down on each other.
Now then, I've noticed that some discs kept in the "Keeper" wallet have got a semi-transparent whitish coat on the writing side of discs, obviously an imprint from the backing material. The discs in the Case Logic case do not. The discs with the imprint on them are still readable somehow but it seems like they read slightly slower, as if the drive is having a harder time reading them.
It seems as if the backing material has somehow either melted off or chemicals from it have left that imprint. I have read the various posts on how heat can cause the plastic backing material to give off certain corrosive chemicals so I know that argument. I used to put discs which were still warm after a burn into the wallets. I've noticed that when I let the discs cool completely before putting them into the "Keeper" wallet, they have little or no imprints on them. So that's one word of advice : make sure you let the discs cool off first before keeping them!
I've also found a way to remove the imprints. Using lighter fluid (Zippo or Ronsonol) and tissue, I managed to rub off the imprints. You have to rub quite hard and some brands of tissue paper are quite rough and will scratch the discs badly. See if your discs have some sort of imprint looking stuff on the recording side of the discs, if so, maybe you can clean it off as I have. Try on some fairly unimportant disc first, though just in case it damages your discs. Mine work fine after cleaning but it is better just to be safe.
Oh yes, and so it also seems it matters what wallet you buy. Needless to say, stay away from the "Keeper" wallets I have used. They are quite cheap which is why I bought them. The Case Logic ones are bloody expensive; but they seem to be safe.