Does orientation during storage affect CD-ROM or DVD lifespans?

Yes, it matters, but not for the reason you're thinking. According to the NIST:

The disc should be stored in its case and placed vertically, like a book, on a shelf. Long-term horizontal storage, particularly in a heated environment, can cause the disc to become permanently bowed. (from §5.2.6)

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?

It is best to purchase new CDs/DVDs as they are needed. According to the Optical Storage Technology Association (OSTA), the unrecorded shelf life of a CD-R/DVD-R disc is conservatively estimated to be between 5 and 10 years. Source: http://www.osta.org/technology/cdqa13.htm

6) How long can I expect my recorded CDs/DVDs to last?

CD/DVD experiential life expectancy is 2 to 5 years even though published life expectancies are often cited as 10 years, 25 years, or longer. However, a variety of factors discussed in the sources cited in FAQ 15, below, may result in a much shorter life span for CDs/DVDs. Life expectancies are statistically based; any specific medium may experience a critical failure before its life expectancy is reached. Additionally, the quality of your storage environment may increase or decrease the life expectancy of the media. We recommend testing your media at least every two years to assure your records are still readable.

11) How should I handle CDs and DVDs?

Handle discs only by the outer edge or the center hole, never by touching the surface. Fingerprints can disrupt the tracking of the laser on the disc. Follow the manufacturer's instructions to remove any dirt, fingerprints, or smudges.

13) How do I store the discs to extend their useful life?

Discs are best stored upright (like a book) in "jewel" cases that are designed specifically for CDs/DVDs. Ideally, store the cases in plastic or steel containers manufactured specifically for the type of medium in cool, dry storage that is free of large temperature fluctuations. Generally, useful life will be increased by storing discs at a low temperature and low relative humidity, since chemical degradation is reduced in these conditions. Store at 62-70 degrees F. and 35-50% relative humidity. Fluctuations in the storage area should not exceed +/- 2 degrees F. in temperature; relative humidity should not fluctuate more than +/- 5%.

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.