It depends on how you want to fly. Do you want max throws? Medium? It is also very nice to have more up then down when giving roll input (the surface that moves up moves further than the surface that moves down). Do you want your plane to have a wicked roll rate (high aileron diff) and a calm elevator (low) or visa versa? It depends on your flying style. Set up the mixing as per the manual of the plane and if you don't like it like that, you can change it up.