I have to disagree. Differential should be set on up or down-lines when the wing is not producing lift.

Ill even make a bolder statement. It is not possible to have a perfectly axial roll while the wing is producing lift. Why? because a symmetrical wing must have an AOA in order to produce lift. And that AOA changes between upright and inverted flight. Sooooo, on a symmetrical wing, both ailerons should move almost identical amounts.