When you get an intact tank, have a look at the bottom of it. Many of the newer tanks are designed with a plastic frame that holds the bottom glass a fraction of an inch above the surface that the tank sits on. That means that the slight imperfections in the table or stand will not put too much pinpoint pressure on the bottom glass because it won't touch the glass. That kind of tank should never have anything between the tank and the table or stand that it sits on. Some other tanks are made with the bottom glass designed to sit directly on the support. They do not have any frame that holds the glass from making contact. That kind of tank would benefit from using a thin layer of soft material, like the polystyrene, to absorb the imperfections in the stand and prevent the resulting pressure points being transmitted to the bottom glass itself. Another thing to avoid, no matter the design of the bottom and frame, is any heavy object in the tank placing a concentrated pressure point on the inside of the glass. I always put rocks and similar items above a layer of gravel or sand so that the weight of the rock is spread by the sand and does not create a stress point on the glass bottom.
In summary, it all depends on the tank's design. Look carefully at the bottom to determine your situation.
