So long as the blocks are dry, and stay that way, it should not be a problem. If any of the wood is wet or water gets in, mold is a possibility. Stabilized wood is pretty much a block of plastic, any mold stain should stay on the surface. But un-stabilized wood will absorb water from humid air. Unless the Styro box is in an air conditioned space, I'd only keep the stabilized wood in it.