Are you talking about developing a flat spot on the tires, or actually losing air from the tires? Most tires will lose a little bit of air over a long period of time, so I don't think putting something under the axels will prevent that. I would think that the only way to prevent the tires from developing flat spots would be to raise them completely off the ground.
2016 Eagle 323 LKTS
2009 GMC 1-ton Dually Diesel