Related: Does orientation affect hard drive lifespan?
I have been told that optical media must be placed in the horizontal position during storage, the theory being that gravity can affect the particles in the media material between the polycarbonate (plastic) when stored vertically and make it go bad faster than it normally would. I probably have the time to test this theory, but don't want to wait that long for the results, if they come in time.
I think it doesn't matter at all.