The only reason I could see you needing a spring would be if the geometry of the system changed at different points while turning. This would cause the cable to tighten up or go slack as the steering wheel/rudder moved. From what I see, the control arm on the rudder with the round groove should have a constant radius over the full movement of the rudder. The only question is if the cable stacks on top of another layer as you turn the wheel which would cause a change in radius there? If not, then I think the original designers got it right.
I fly R/C airplanes, and sometimes use a pull-pull cable set up on the rudder on an airplane. If you don't have the same dimensions at the servo and the rudder, the cables can bind up or go slack. Even with careful measuring, I typically add a small spring to one side of the cable just to be safe since a stalled servo can draw a lot of current and drain your battery.
There is a lot of discussion about cables on boat rudders, and a broken cable will cause a loss of steering. I was riding on a friend's Ski Centurion back in the summer and he was showing me how maneuverable it was. When he turned the wheel hard, I was on the other side of the boat in the blink of an eye!
I'm not sure what would happen if a cable broke, if the rudder would tend to self-center or not, but I would make sure your set up is solid and have a routine maintenance and inspection routine.