For me, the reason I will always use something under my tanks (all glass) is that I can't trust the stands/furniture they are on to be 100% flat. Wood will sag slightly, more so over a longer distance. Glass does not sag, so if you have a wood surface with a glass container directly above with a LOT of pressure, this MAY cause a crack. I use polystyrene foam for the same reason I like suspension and inflated tyres on my car. If I drive over a pebble, I don't want the car to fly up into the air and the axle to break. Glass and acrylic are not all that flexible... if you have a filled 6 foot long glass tank on a hard surface with a 2 millimetre grain of gravel underneath the centre of it, and nothing to remove the strain, you will get at least stress fractures, at most, a crack. For me, this is the application of physics with the addition of a few years of experience. Not saying this this is a definite, just asking you to consider the possibility.