As long as a CD/DVD is not used to support external weights, it hardly matters whether they are stored Horizontally or Vertically. Theoretically, a CD/DVD kept horizontal supported only at the ends can deform due to creep - over a period. The point is, this period is long enough to make the contents obsolete even for the archives.
Keeping them cool and in dark enclosure is the best way to preserve for a reasonably long time - say 6 to 8 years. I don't think any other means can guarantee infinite life.

Ramesh