For most people, Rodney is correct. At center, the cables are both tight regardless of the setup. When pulled to one side, the airflow from the plane's forward motion and/or the prop blast keeps tension on the pulling cable and the offside cable has nothing to do.
However, if your flying style involves tail slides with the power off, the reverse airflow on the rudder could cause the rudder to pull up the slack on the offside, thereby slightly increasing the rudder deflection. So there is one (admittedly unusual) situation where you might want to have both cables taught all the time.
Jim
That would be an extreme case however having the control system loaded the same on both sides really does give a more precise control response if you are flying an airplane of high enough performance level and have the skills to match. For the average guy flying his Sunday favorite it may in fact not really matter but some of us do push things to that next level or two. Rodney, try flying an airplane with over 300 sq in of rudder area through a 20' high 8 point roll with one cable going slack and you will soon find that it does indeed make a difference.