I work in Flight Controls at Boeing, and as an avid Aero Enthusiast have learned all that I can, and also used my head. There are sevearl factors that Rodney and Alasdair are correct on.

2 forces are acting on the moveable surfaces, ailerons, rudder, flaps, spoilers, horizontal stabilizer, trim tabs etc.

The first force creating a moment at the hinge line is the mass of the object itself.
The Second force is the air moving over the surface creating moments, drag etc which vary depending on the airspeed.

On manual aircraft (almost all aircraft made untili FBW Flight-by-wire aircraft) the surfaces and controls were set with the pilot in mind so that the stick force could actually move the surfaces.

ON Flutter. Flutter is directly proportional to the stiffness of the structure, Stiffness of the controling mechanism, Velocity of flight, and mass of the surface.

The natural frequency of an object is directly proportional to its mass and geometry.

Rodney is correct in saying that if the structure is sufficiently stiff so that all the natural frequencies are out of the operational range or harmonics which are almost just as bad then no counter balance is needed. In fact if you add weight then it can simply cause different harmonics/natural frequencies to be induced.

Alastair is correct in that mass balancing can solve some natural frequency issues but its effect is simply that of a "larger" force being needed to induce the same resonant displacement. Qualifier: this is not necessarily true in all cases. Most structures resonate at very low frequencies, thus if you can lower the natural frequency below the excitation frequency, air loads, or possible engine vibes then this will fix the problem.

Full sized aircraft, 2-seaters etc, are mass balanced because most planes are designed for 50lb stick force max and 80 for rudder... there are FAR's about maximum allowed stick force/rudder pedal force. My numbers are close I think but not correct. This is also why you see Z rudder tops and horizontal stabilizers, because they act to counter the moment needed to move said surface.

For instance on all of your full scale commercial aircraft such as 787,777, A350, A380 they don't mass balance anything, but rather design the structure to be sufficiently stiff. IN otherwords the attachment spar and the controling mechanisms. Saves weight thus more efficient airplane.

The quintessance(<<Spelling?) of this principle is observed in variable pitch constant speed propellers where the propeller has to balance the aerodynamic forces, mass of the blade, and be very stiff for most efficient acceleration of the propelled mass of air. In this case the stiffness of the propeller blade often overides any natural frequencies that may be induced by engine vibrations. Off subject, but the principle still applies in a rather obfuscative measure to be sure.

Brian Foote
PS flaps on small planes are usually not counter balanced becsaue they are designed for one operating airspeed (generally), thus you can balance the aero forces and the weight, but this is usually discarded since anymore most private aircraft have a servo mechanical device deploying the flaps or a deploy assist.