Let's be logical about this dilemma:  does it make sense to store
things vertically or horizontally?  I would posit that, on a case by
case basis, one decides based on many factors: (1) condition of discs,
(2) size of discs, (3) number of discs, (4) frequency of use of discs.

First off, it makes no sense to worry excessively about fine, sturdy,
undamaged discs of any size, so store them in snug vertical dividers
and be careful getting them in and out.  If they are fragile, all bets
are off; best not combine rusty, bent, dinged and warped discs with the
ones that are mostly perfect.

Secondly, the small ones don't have as much potential to warp, so the
larger the disc, the more care taken.  For example: 4.5" discs are fine
in a little box, 8" discs work well in a file cabinet with bogus paper
dividers, 12" discs fit handily in sturdy accordion file folders,
15-1/2" discs in a traditional vertical storage cabinet with the
addition of nice sturdy not too thick paper dividers to pad them, as
long as one does not cram too many into a single space.  With this
system no problem seems to occur with sizes up to 20" or so.  Somewhat-
to severely-damaged discs are segregated elsewhere, if you want to keep
them at all.

Thirdly, how does one catalogue a large number of discs, much less
store them?  Which leads to #4, how often do you want to play these
discs?  Can you imagine wanting the one on the bottom and how to get
to it?

Cheerfully,
Claudine Jones
San Francisco