Based more on my intuition than anything:
Not in the basement (unless it's sahara dry), not in the attic.
Nowhere where there's large swings in temperature and/or humidity.
The humidity part is pretty obvious to most, and temperature swings will affect relative humidity as well.
Remember that different materials have different coefficients of expansion, and ammo is a collection of different materials.
To maintain a good interface between bullet, case, and primer, I'd avoid exercising them thermally.