Re: Storing on blocks
IMO it is always a good idea to put a stored vehicle on blocks, especially for the tires, and especially if they are Michelin which seem to be more prone to cracking sidewalls.
Springs do take a set from static load over time and will sag, so getting the load off them will help them. If you have airbags, inflating them to near max will give the springs a break and not hurt the bags. If you are concerned about the fully extended suspension, the normal method is to support the vehicle by two stands under the differential housing, two stands under the front a-arms, and then lift the body a bit on and put the normal 4 stands under the frame points. This gives you the best of all worlds, reduced load on the springs, etc, and no full extension of the the suspension.
If you don't put it on stands, you should pump up the tires to max rated pressure, and if possible, drive it a bit once in a while to get the tires in a different spot.
Plastic under the entire vehicle is a good idea if the area is dirt, or damp concrete, to keep moisture from condensing on the undercarriage, but I don't really like it under the tires as the plasticizer in the plastic can migrate and make them stick sometimes.