Positive expo is rarely used... and as mentioned, not all radioes have this option.

Since the servoes move in a sirkle we in fact have positive expo to the rudders if we don`t use expo-settings in our radioes! This is not known by all pilots i think So if you set the expo (negative) to 10-15% you get a linear ruddermovement, and if you increase the exposetting you soften the senter.

For my normal planes and sailplanes I use 10% on all rudders, and for 3D I prefer 50% to the elevator, 40% to the ailerons and 20% to the rudder where I do not want softening. As you might understand: the 10% setting is just to get linear movement, and for 3D I have found my preferances. These are individual.