Rodney maks a good point. I actually intentionally use offset geometry (piviot slightly aft of hinge) in my pull-pull setups for the very reason he indicated. I.E. line not pulling goes a bit slack as surface deflects, both lines taught at neutral. This works well because at deflection, only one line is working under tension, i.e. the pulling line, the slack line does no work. However, around netural, both lines go taught giving good neutral deflection control.

jaka makes a good point. With very long control horns and short servo arm, the "gearing" is such that the effect of line slop is reduced, so lines don't need to be as tight as a symetrical system to have the same precision. However, this method will not work for planes that need large throws, i.e. 3D. But would work very well for general sport, precision planes, etc.