I'm not sure if there's any official advice from manufacturers, or a generally accepted answer, but here's my two pence:
- short answer is it probably doesn't matter; long answer:
- If it's for a really long time, it may be better to keep them upside down so the oil in the lower lubrication bath is keeping the seals oiled. I've read claims that seals can harden and degrade if they're left dry for a long time. I'm not sure how much I believe this though
- for example I've got some ancient sets of marzocchi forks where this has never been a problem, including some 888s that are over 15 years old, stored attached to a bike rightside up and still work fine with the original seals and occasional oil change on the odd occasion I take my ageing downhill bike out for a spin (pre Fox marzocchi forks had a well deserved reputation for needing next to no maintenance though)
- personally I wouldn't worry about this too much, but when you come to use the forks again maybe flip them upside down for a minute or two to get some oil into the bushings. Even better would be to pop the lowers off and put some fresh oil in. Better still, replace the seals while you're at it.
- I've had issues with rear shocks stored for long periods where the damping oil has leaked out, but this is with fox shocks where the damping circuit is charged to around 300psi. I'm not 100% sure but I think with a turnkey damper the oil is just at atmospheric pressure when uncompressed (at least that was the case last time I did any work on a RS fork circa 10 years ago)
- can't really think of any issues related to leaving the air spring pumped up or deflated. It might leak slowly over time, but you can just check the pressure when you come to use them again. I don't know if there's any risk of some plastic deformation of the o-rings if they're subject to normal spring pressure without moving for a long time. I guess you could let the air out.
- a long time ago I used to work in a bike shop. We stored new forks flat in their boxes, display models rightside up and never gave it any though (though that doesn't mean it was ok). Forks attached to a bike where inevitably right side up
- I've got frames that have been left for years in damp conditions and the paint has started to blister and flake off. Not seen this happen to any forks, but if it's going to be a really really long time might be better if they're somewhere not too damp.
- all of the above is probably over thinking things
My experience (and that's all it is) is that the seals perish and that oil will leak out of a fork on a bike hanging unused for a while (a few years) by its front wheel. New seals fitted at not great expense by LBH, which commented that they had just crumbled. No idea what difference regular use as opposed to disuse makes to life expectancy. Fox, for what that's worth