I've heard several seasoned xc skiers recommend putting on a coat of wax on before storage for the season. I don't think the temp of wax matters, just that the surface of the ski is protected, as another person mentioned, the dust, etc can ruin the ptex.
It doesn't really matter if it's flat or upright. Important thing is to protect ski base with layer of wax which is not scraped away. With this you protect ski base from dust, oxidation and any other damage it might happen during storage or transportation, and you keep your ski in perfect condition.
When I was still in top level xc skiing I had all my skis prepared this way for every transportation between race places etc. It takes some time to get 10-20 pairs of skis prepared, but it's only way to keep your racing skis in good condition.
By all means, aply a double coat at season end to protect the bases, but more importantly, keep them away from concete floors or cement block walls. You can oxidize the rails very easily by putting them in contact with either. They'll pull moisture right out of the floor/wall if you do and they'll be covered in rust. See it all the time at the ski shop.
I'm not much of an xc guy, but I alpine ski like its my religon and I have found that a cold wax works well for storage because it is very hard. Tough to get rid of come fall, but is very protective, make sure you get the edges coated too, it will keep away the rust (if you have edges)
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