I have recently built an aquarium setup, including a plywood tank. I didn't use any styro for this setup because, similar to acrylic, the water weight is applied across the entire bottom surface. In glass tanks, where the outside edges support all the weight, it's wise to use something for leveling (actually further weight distribution). Even in professionally built cabinets I'd use 1/2 styro for large glass tanks. I own a 55 on a pine store-bought stand, and didn't use it on this, but anything larger I would.
As far as styro (of even a solid cabinet top) insulating the bottom of the tank, I'd have to agree with those who say that there is minor loss of heat through the bottom glass. Let me clarify a bit, it may not lose heat "through" the bottom, but cooler air below it causes the bottom of the tank to get colder. Just a simple experiment would be to put a glass of water with a solid (gasket sealed) lid on top and see if the water freezes (assuming the top won't be forced off in the process
) The water is not just loosing heat through the top, but is being affected by the cooler air all around it. Even cakes are laid on those racks that lift them off the counter-top to cool down quickly and evenly after baking!
I've also read a few articles by die-hard aquatic gardeners, who were discussing substrate heating cables. The ones with open top stands and glass tanks noticed differences in later tanks with solid top cabinets. Acrylic worked better, and heating cables with solid top cabinets worked the best. A good way to test this for yourself is to get two identical setups but with an open top stand and a closed top stand. Use glass tanks, same substrate and basically the same plants (aquascaping matters little), and heating cables with the same wattage and a thermostat. If you can see when the heating element is operating, it will turn "on" more often in an open cabinet top tank. You will need the room to be 5 or 10 degrees cooler than the tanks to be able to effectively notice. I would say many of these aquarists who have operated these tanks and taken notes are the best authority on this heat loss topic!
In closing, I'd say it can be a useful investment to use some styro, for the purpose of both added padding/leveling and additional insulation. It's fairly cheap to buy the 1/2" sheets and it can only help your tank. I can't think of another drawback except minor cost of insulation.
). I prefer this type to expanded polystyrene, it's easier to cut. As far as leveling goes, every stand is dead level, so while it may not be entirely necessary, it gives me peace of mind 
