Either way, but keep it out of the light unless you plan on dyeing everything you make. The sun will "sunburn" vegetable tanned leather fairly quickly, artificial light is much slower but I think will still end up causing a reddish color. When I was doing leather work for a living we kept the weights of leather that we used a lot laid flat in a storage rack but we still tried to keep it covered with cardboard, stuff that didn?t get used often was kept rolled up in boxes. I saw one shop that used large diameter PVC pipe to store their leather.
Todd