I think that with something like the typical 6PPC Benchrest rifle, that if you have a straight FL die/bushing combo, or a one piece die that is the right size for your chamber, and are running typical neck clearance, with the bullet seated so that the rifling marks it, that rounds are straightened to an acceptable minimum runout when they are chambered. In my rifle this is .0015 measured about 3/16 in front of the neck. IMO concentricity gauges are best used to diagnose equipment problems. Once a set of dies has passed the test, there is little reason to use one, except to occasionally check to see if anything has changed. If the cases are straight from sizing, and the fit is good on one that is neck sized, the little bit of difference with a FL sized case shouldn't matter. A properly fitting FL die should not change the size of the case much at all. Of course all of this is predicated on the use of good brass.