I used untreated plywood for my previous layout. When dismantled after about three years it showed no signs of warping and some of it will be used in the rebuild - again untreated. Having said that, my garage is part of the bungalow and perfectly dry and does not have too much temperature variation. This is because it has double-skinned walls with insulation, a pitched roof with plenty of insulation, and the car door permanently closed and sealed. In effect it is just a spare room of the bungalow.
Robert