For those who balance on simple pivoting device (store bought or homemade) here are some examples of different airplanes that were either checked/balanced right side up or inverted as needed. The device here is the old Sig kit balancer currently discontinued which has ball bering pivots.

Each of the airplanes are stabile enough to be unsupported in the pictures and most cases its easy to see that there is sufficient mass of the airplane below the actual pivot point to allow it to balance without holding.

John