My 21 is sleeping, too, but it's not covered. Should it be covered (against snow etc.) ?
Covering the boat mainly just protects against UV damage- gelcoat fading. If you live in an area that gets heavy snow, be sure the boat is well supported on the trailer, otherwise excessive snow weight can damage the hulls. Also, if you "tent" a tarp over the mast, the snow weight can damage the mast.
And...I've heard some people talk about fiberglass soft spots. I checked my hulls and no soft spots...but what causes soft spots? Is is a rot or storage-related thing?
Soft spots are caused by a break down of the fiberglass/foam sandwich. Either the fiberglass delaminates from the foam, one of the fiberglass skins cracks, or the foam deteriorates. This can be caused by a number of things. A strong impact (like ramming the side of the boat into a dock or jumping up and down on the hull) can cause the fiberglass skin to crack or delaminate from the foam core. Allowing water to remain in the hull for long periods can allow moisture to eventually penetrate into the foam core and cause the core to break down which is why it's important to keep the inside of the hulls as dry as possible.