Nice points Bill. If I may expand on Bill's comment about minimizing distortion.

That point is that the distortion products multiply, a second harmonic combines with a second harmonic in the following stage, producing 4th harmonics. If we can minimize one of the second harmonics, then the resultant 4th will also be minimized.

This scenario really gets ugly with multiple stages as we start dealing with higher orders.

Hope this helps.
Steve