I wouldn't recommend ever keeping them in any kind of bag. I don't even keep any of my DS V's in the plastic tissue that they've been coming in the past few years; I fold it up and put it in the corner of the box. Ever watch Indiana Jones: Kingdom of the Crystal Skull when bodies are unwrapped that have been airtight for a long time, and they turn to dust in 30 seconds? That kind of disintegration won't happen before your eyes in reality, but the concept is still the same. The lack of oxygen in an airtight bag, or like when people shrink wrap their shoes, will cause the shoes (especially if they have polyurethane elements) to break down a lot faster than if they are just stored in the box. The restriction on UV light is the real key detail in storing and maintaining these, but there are other factors too, like local climate.