Hmm..never thought about that much, but you do have a good point. I'd think the crucial thing is to prevent any deterioration of the rubber parts. Cool, dry and away from any damaging UV rays.(think of what the sun does to your wiper blades every year!).
Bagged, and with a dessicant pouch or two in the container? Also making sure that they are not folded which can place a crease (think bad seal)or start a weakness in the rubber. Perhaps using something inert to hold/reinforce the correct shape of the face piece would be do-able if you really wanted to be extra cautious.