The best rule of thumb I can offer is; if you're running steam era 40' cars, superelevate. If you are running modern, tall, high capacity cars ( 89' auto parts boxcars, auto racks, double stacks) don't superelevate.
I know it looks great, but the modern cars will tend to fall into the curve, and superelevation ( enough to be visible ) will increase that tendancy. Remember, model trains do not have the same physical "moments"( distribution of weight, center of gravity, reaction to centrifugal force ) as the prototype cars, and react much diferently to superelevated curves.
Pete