Sub-trim should be used to center the servo wheel in relationship to the pushrod, not the control surface. This way you geometrically get the same amount of throw on the rod from the servo in both directions as opposed to setting the servo up with differential. You should first try to achieve this mechanically and then resort to using sub-trim for minute adjustments. An exception to this might be the throttle where you are attempting to center a mechanical trim lever and achieve a proper idle speed. Control surfaces should be centered using the clevis at each particular horn.