Thanks for confirming what I was trying to say, Jim. Whether the rudder is forced to "offset" with a tab, or spring tension, the result is the same. With spring-tensioned rudder cables on some airplanes, I've been able to adjust cable length a small amount, with turnbuckles, to allow the ball to stay centered with my feet off the pedals in cruise, without having to add a trim tab. Of course, it depends on the strength of the springs, & you can substitute a stronger spring on one side also. You can't do that with an RV-6 (no springs).
I agree, Lowell, that a trim tab is best, especially if you need a lot of rudder offset. My choice would be to add a tab, and adjust cable length to synch pedal position if you're sensitive to that. I guess if you just need a little adjustment the turnbuckles could be more convenient/accessible than the multi-hole links at the cable/pedal attach points, but I see your point.