You cannot prevent it, only reduce the likelihood or severity.
The best wood, prepared by the best cue maker stored in perfect climate control can still move.
Will a sealed case help? Maybe a little. I would say it's more important to store them in a stable climate controlled setting.
The seal of such cases isn't exactly really totally sealed anyway. You can hear a pop when they open because you open them faster than the seal can leak.
Wood will slowly oxidize with age. That is a natural process and it is the reason it gets darker. The natural aging of wood cannot be prevented. It is possible to slow it down. A sealed enclosure filled with something like nitrogen could do that. These would be measures taken by museums to preserve valuable paper and wood items.
To me, the most prudent storage is in a climate controlled area that is stable. Ideally that is in some sort of enclosure that is "relatively" sealed and not against any exterior wall.
Some people get very uptight when their shaft moves a little, I don't, to me it's normal. Check them carefully as the seasons change and you will likely note the movement cycles with the seasons because of slight temperature and moisture variations. In one season a shaft might be perfectly straight while in another it is not. Or it might always be a little off but to varying degrees.
Buy the best shafts you can and take the best care of them you can, but you cannot with certainty eliminate the movement of the wood, only reduce it or minimize it.
That's how I see it anyway.
By the way I do prefer tube cases for storage anyway, like the GTF/Fellini, Centennial,/Thomas/etc. I think such cases are, bar none, the best type of case for pool cues in general anyway.
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