I often see this as a requirement but; it is not. As long as, when the servo is at full throw, no binding occurs. The horns can be either longer or shorter than the servo arms if there is no negative Ackerman in the system. I always like a little positive Ackerman in mine (where the non pulled line gets a bit slack as the surface is moved away from neutral). This slack will not hurt a thing and; no, it does not contribute to flutter in way, shape or form nor detract from firmness of control. It is also the easiest on your servos as there is no current increase as happens if the line tightens under movement.