Grayhawk,

Control surface balancing has nothing whatsoever to do with gravity or with surfaces hanging level when mounted.  Gravity hinge moments are (a) small  (b) steady   (c)  in the case of ailerons and rudder, invisible to the pilot  (d) in the case of elevators, trivial at most and can be taken out with the trim tab as desired.

Control surface balancing is about making sure that if the surface starts to flap up and down at very high speeds and dynamic pressure, when vortex shedding frequencies start to match natural frequencies of the structure, it doesn't through inertial coupling excite a corresponding motion in the fixed surface it's mounted to (wing, vertical stabilizer, horizontal stabilizer) and drive the whole collective structure unstable.  The best way to prevent inertical coupling is to have the CG of the moveable surface exactly at the hingeline -- neither above, below, ahead or behind.  That's possible in a centerline hinge, not possible in a hinge that's on the top surface.  The best compromise for a top-surface hinge is to have the CG directly below the hinge.

CB