It depends on how the tank is built. Flip it over, and put a straight edge across it. If there is a gap between the bottom glass & the straight edge don't use any polystyrene. Doing so will put pressure on the bottom glass, which it is not designed to take. This could result in the bottom glass breaking.
If the straight edge does touch, it will need polystyrene. This design is made so that the bottom glass will take the full weight. Any small piece of debris will create a very small, but very severe pressure point without the polystyrene.