Today's rockwool that is manufacutred for gardening is made to keep the optimum amount of oxygen in the fibers, even when the cube is soaked. You should be fine, as long as the cube isn't getttng light. I have had problems with algae and fungus colonizing my rockwool, but if no light then you are good. That stuff is all purpose, I have yet to see someone find a way to misuse it.
