Robbie asks a good question about the weight that discs can hold
without bending the projections and, unfortunately, this is a question
that I never want to discover the answer to.  I can attest that they
can support at least 75 discs in a horizontal stack without damage,
as I've got that many in one pile.

As to which way to store the discs: All of the disc storage bins
that I've seen that were made for disc type machines store them
horizontally, which seems to indicate that this is the best way to
store them.  Practically, when I want to select a specific disc it's
much easier when there is no weight on it to pull it out.

A couple of cautions, however: if the discs are stored horizontally
there must be a support to make sure that they don't roll out and fall
on the floor, as this can surely damage the disc.  Discs larger than 11"
have an outer circle of holes to drive the disc and this rim is easily
bent or broken.  (It be repaired, but better to not let it happen.)

That said, I currently stack mine horizontally, but only because I
haven't got the time to build a proper storage case.  Note also that
they will take up more room if stacked horizontally, because you'll
want to remove an individual disc which necessitates an amount of space
between them.

Eliyahu Shahar