Even if we think about symmetrical airfoils, this is very relative, the curvature of an airfoil is a physical measure, you can say it is identical on both sides, but in order to generate lift, an airfoil needs to have more curvature on the top of the wing(I mean surface looking up, so it doesn't matter if it flies inverted), because this is a fixed value then AOA(angle of attack) will increase the curvature, so even if your wing and stab are at 0deg on a symmetrical airfoil, in flight it will be positive AOA.

So inverted or upright will need the same differential, but during verticals, the prop wash or P factor also affects the differential, also the engine torque might affect this, there are airplanes that depending on the design, it will not react as desired, so, some times after differential has been adjusted to one side you need to adjust to the opposite direction with ATV.

When adjusting differential be careful not to use too much that it will actually change the heading.

In a few words, don't stress, it is not magical or perfect, just adjust until you get the best roll reaction possible without changing heading(you can see this during verticals) and to the usual upright rolls.

I hope this helps.