A tank with sides around the bottom can only be used without foam, if the bottom is raised up inside the sides...this is full floated, and it is similar to a rimmed tank. A semi-floated bottom (bottom in contact with the standtop, flush with the bottom of the sides) needs to be on foam. Both semi-floated, and sides on top of the bottom require full bottom support and foam. Full floated tanks, can be supported by a rim only, and can be "treated" as a rimmed tank, so to speak.
There are other consideratons however, when dealing with rimless tanks, that would determine whether or not it is "safe to use."