As others have said, it couldn't hurt to put them up on wood. However, it's probably not as important as if you were storing them on an asphalt surface. Some folks use 2X8s but I go with 2X10s because it's easier to center the tires, making sure that they don't hang over the edges. Also, make sure they are inflated to their maximum pressure.
2012 Eagle Super Lite HT 26.5RKS
2005 GMC 2500 SLT HD D/A