If you use the procedure describe above by Danny, you will see that virtuallyall models need at least a bit of differential, and some need a lot in order to roll axially.

Keep in mind that when ailerons are deflected they initiate not only a roll but also a bit of yaw opposite the direction of the roll. Aileron differential compensates for this yaw by creating a little - or a lot - more drag on the "up" aileron, pulling the nose of the model back to the intended heading.

I have a Seagull Super Decathlon (some similarities with Cub) which needs 50% aileron differential, and even then aileron rolls are not quite axial. This is a straight wing (no dihedral) with semi-symmetrical airfoil. There are three reasons I can think of for this flight characteristic: 1) It is a high-wing design, so the point of rotation is well below the wing; 2) the fuselage has a huge amount of side area, which exacerbates the yaw component when it is presented; and 3) the model has a very short tail moment which reduces its tracking ability.

I'm sure there are other (perhaps many?) such as the wing's AOA, nose moment, CG, etc. etc., but it is easy to see that even the best design probably needs at least a teensy bit of differential.