ORIGINAL: grotto2
If you work through the geometry, you'll see that in order to exactly duplicate the movement at the control surface it will need a round control horn. Otherwise the take-up side of the wheel pulls more cable than the other side needs reeled out.
The linkage system can be symmetric even with classic horns, you have to pay attention to the clevis point and control horn placement to get this correct.
Contrary to the common belief, it is also false that the servo arm and control horn should be of equal length in order to avoid slack cables: in my plane I computed the arm length in order to have a given amount of throw, say 40-45 degrees: with the servo travelling at +/-60 deg, this way we can have a greater mechanical advantage.
Very good linkages tutorial, a must read for pattern airplanes, is available at centralhobbies site: thanks to Troy Newman!