Talking about long term seasonal storage here. Short term like camping and such, don't sweat it.
Storing outside, you want good drainage so the tires don't sit in puddles which can degrade the rubber over time. Also the puddles can freeze, a worse condition yet.
Another good practice is to store the rig with the tire pressure at the max as stated on the tires. For most "E" rated tires, that is 80 psi. Doing so will minimize contact to the ground, help in maintaining roundness, and you'll have a better chance of having enough air in the tires in the spring time to drive the rig out from the storage lot. It is also good to cover the tires from the sun.
As far as wood versus plain old ground, I vote for patio blocks. Over time, wood has the potential to warp into a bowl shape on soft ground.
What I always practiced with my Corvettes, is not practical with a motor home. It is ideal to jack up the vehicle so the tires are suspended in the air, done indoors in heated storage. The springs are relaxed that way too. Getting even pickier, open all windows and doors a little (unlatch the Corvette convertible top too) so all the gaskets are relaxed for a better seal during the driving season.
