Concrete is the better choice as asphalt has petroleum based products in it and after a time of just sitting the tires can be damaged (what I have read). So if it is asphalt, use 2 X 12 X whatever length. The important thing is to keep the tires evenly distibuted on the boards. Do not have any of the tire overhanging the board.
Even on concrete, for extended periods of time, I would use boards. Only because they elevate the tires from someone spilling a substance that can be detrimental to the rubber.
I figure that these people know how to store there tires. This is from the Michelin RV Tire Guide.
"LONG TERM STORAGE AND RV TIRES
Unless the RV owner is a full-time RV-er, the vehicle
probably spends some time in long-term storage. But what
the RV owner probably didn’t know is that rubber tires age
when not being used. So, if the owner must store the RV,
a cool, dry, sealed garage is the best bet. Also, some storage
surfaces can cause tires to age faster. That’s why Michelin
recommends placing a barrier (cardboard, plastic or
plywood) between the tire and the storage surface.
Here are some other steps the RV owner can take to help
reduce the aging effects from long-term storage:
1) Thoroughly clean tires with soap and water before
placing into storage.
2) Cover tires to block direct sunlight and ultraviolet rays.
3) Store out of a high ozone area.
Note: When a vehicle is stored, tires should be inflated to
the inflation pressure indicated on the sidewall.
Before removing the vehicle from long-term storage,
thoroughly inspect each tire — this includes sidewalls, tread
area, and air pressure. If the tires have lost air, be sure to
inflate them to the correct pressure before driving."
You can read some more on tire maintenance at: Suite 101
Notice that Goodyear states you can go 25% over pressure, but continue to read because it state to NOT exceed the maximum pressure. So if your weight is such that your tires are inflated at 82 pounds, and the maximum is 110, then you can inflate to 110, but if you are inflated at 95 pounds, you can still only inflate to 110 pounds if that is the maximum inflation pressure.
Edited: If you do decide to store with jacks down, before taking them up, wipe down the piston with a clean rag and some transmission fluid. It will get any crud of that has accumulated while in storage.