[drcy]

No universal rule, as its a different situation for different cards. A holder won't stop damage. However, for many Pre-War cards, the damage is from a long time ago, and as long as the card has been dry in recent years and is dry now, it's likely okay. If the damage does appear to get worse and/or the mildew or whatever is from recent bad storage, then you can have the card treated-- there are methods to fix it. Keep the card dry and keep an eye on it. If the professional grader holders hold the card in acid free material (I don't know what they use, but assume they use good materials), that's a good thing for helping preserve the card.

Foxing, or those brown old age spots, are not uncommon on old ephemera. The good thing about it is it is good evidence the item is indeed old.