Most of the counter balances on RC planes are of the aerodynamic type (and mostly for looks) the aerodynamic counter balances on full size are used to reduce the stick forces (or RC servo loads). Mass balances on the other hand are used to prevent flutter. Its done like its described in the previous posts. The only rc plane I've ever seen that recommends them is the US Air Core Colt, and they recommend using them on the ailerons. However they do benefit other rc planes. Me and a buddy of mine built a plane called the "Fun 15" that has massive control surfaces that are aerodynamically balanced, mine's glow and his is (was) electric. I make it a point to keep the speed down when the nose is down, he decided to try a power dive. The resulting elevator flutter turned it into a lawn dart. I added lead to the leading edge of my counterbalances to mass balance the sufaces and with the nose down and power on there's no sign of flutter. Have a similar small 3D electric that fluttered the elevator at high speed, a little lead on the leading edge of the counterbalances solved the problem.

Mike