Spine is an outdated, debunked theory from the 80s. WAY over thought and discussed. It’s nothing more than a manufacturing anomaly of a tubular shape. Many if not most current builders build on the straightest axis because no blank is arrow straight. Materials, dimensions, components and guide spacing are the keys to a quality rod.
This fact that a rod is not casted or fished on a single plane is one of the reasons spine is irrelevant.