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.
Yes, it matters, but not for the reason you're thinking. According to the NIST:
I recommend reading the entire document if you're responsible for archiving optical media.
NIST Special Publication 500-252, Care and Handling of CDs and DVDs — A Guide for Librarians and Archivists.
I think if it made any difference at all, that difference would be so minute - that the chance of merely its detection would be significantly lower than that of dust particles spontaneously springing into life.
This is an ancillary post regarding lifespan of a DVD:
Quoting from National Archives FAQ on Optical media storage:
5) What is the shelf life of unrecorded CD-R/DVD-R discs?
6) How long can I expect my recorded CDs/DVDs to last?
11) How should I handle CDs and DVDs?
13) How do I store the discs to extend their useful life?
Additional data are here in an excellent experimental report by J.Iraci: "Relative Stabilities of Optical Disk Formats," Joe Iraci, in the Restaurator - International Journal for the Preservation of Library and Archival Material (2005) (http://www.uni-muenster.de/Forum-Bestandserhaltung/downloads/iraci.pdf).
What "particles in the media"? There are no particles in cd or dvd's, apart the ones they're made of. There is no mass transfer while recording them.
I guess discs if stored vertically in higher temperatures could due to their (minor) mass and temperature deform somehow, but that's really going a little bit too far. I have some cd's almost 10 years old that have been positioned in pretty much all orientations and they still work. Same goes for old vinyl's (40-50 years old).
Conclusion: don't worry about it.