My two acrylic tanks are of the second type that Dave described. The sides sit on top of the bottom. It voids the warranty of some manufactures to NOT use the styrofoam on this type of tank. It would seem like edge to edge would work for either type. The foam is used to provide a flat surface on which the acrylic can rest. Since the weight of the tank/water is supported by the bottom and the not sides, the bottom needs to be flat, and must be supported not only on the sides and front but also in the middle.
This may not be true of all glass tanks (weight supported by the vertical pieces), but it would seem like it would be for larger glass tanks. It is true for all but the smaller acrylic tanks (weight supported supported by the bottom). At least that is what I read before I got my acrylic tanks.
When I was reading Dave descriptions again, I saw the word raised. Not sure about styrofoam for that type. Seems like you would need a flat hard surface if the side and bottom are not flush.
Oh well, too many words.
Eddie