That is something a lot of "old timers" say. The reason they say that is in the "olden days" vehicle starting batteries used to be made with a hard rubber container. This hard rubber would eventually get mini cracks and become porous. So, when placing a battery on the ground or concrete, the battery would discharge through the ground or wet concrete. Nowadays, containers are made from a solid plastic that does not allow any current to flow through it, so the batteries do not discharge, even if they sit in a few inches of water. That is why you will not find your battery having trouble from sitting on the ground or concrete.