Have you actually witnessed a case where any amount of undisturbed ammo "cooked off" (exploded spontaneously) due to any (even an exacerbated) natural condition?
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Didn't think so...
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(Note: I am not arguing that future ammo performance might not be affected by improper storage conditions.)
And so it would be a good idea to rotate it. No sense having ammo which won't shoot when you want it to.