When you remove the expander from a full length die and size a case the case will be as straight and concentric as it will ever be. "BUT" the neck will still need to be expanded, either with the dies expander or a expander die that expands the case neck on the upstroke of the press with the case resting on its base. And remember any dings in the case rim can cause the case to tilt in the shell holder and cause neck runout on the down stroke of the press. This is what makes the Forster die better than other dies because it holds and centers the case neck in the die when the floating expander enters the case neck. Meaning the expander can not pull the case neck off center and induce neck runout.
The quality of the brass and its neck thickness variations can effect case neck runout. I sort my cases with a Redding neck thickness gauge and select the cases with the least thickness variations when full length resizing. And with my Forster full length benchrest die I can easily get .001 or less neck runout.
Just remember a full length resized case has more "wiggle room" to let the bullet to be self aligning with the bore. Meaning the body of the case and the case neck do not touch the chamber walls, and have very little guiding effect on the bullet.
Below was from the shut down website "The Rifleman's Journal" by German Salazar a national long range competitive shooter. Mr. Salizar was asked about "partial full length resizing and he explains why total full length resizing is better.
Reloading: Partial Neck Sizing
by German A. Salazar
http://riflemansjournal.########.com/2010/06/reloading-partial-neck-sizing.html
"Now the last scenario, a full-length sized case in which the neck is also fully sized. There is clearance at the neck and in the body of the case, the closest fit anywhere is the bullet in the throat. If the neck to bullet concentricity is good (although it needn't be perfect), then the bullet will find good alignment in the throat and the case body and neck will have minimal influence. Let's not forget that the base of the case is supported by the bolt face or the extractor to a certain degree as well; this is yet another influence on alignment. As you can see, there are several points from base to bullet that can have an effect. My procedure is to minimize the influence of those that I can control, namely the case body and neck, and let the alignment be dictated by the fit of the bullet in the throat and to some extent by the bolt's support of the base. Barring a seriously out of square case head, I don't think the bolt can have a negative effect on alignment, only a slightly positive effect from minimizing "case droop" in the chamber. Given that a resized case will usually have a maximum of 0.001" diametrical clearance at the web, this isn't much of a factor anyway."
A neck sized case with case thickness variations can warp and cause bullet misalignment with the bore. Meaning neck sizing works best with high grade quality brass that has been inspected with a runout gauge. And cases that are full length resized are more forgiving because they have wiggle room in the chamber.
Below a case with neck thickness variations will have a high probability that the case body also has variations. And when fired the thin side of the case will expand more causing the case to warp and become banana shaped. I found out about this the hard way when reloading for milsurp rifles with large diameter chambers that gave extra room for the cases to warp.
Below Kevin Thomas who worked in the Sierra balistic test lab and now shoots for Team Lapua USA. And please notice he is also a member of the "rat turd in the violin case club".
Bottom line the average shooter with average off the shelf factory rifles with SAAMI chambers is better off full length resizing. And a Forster full length benchrest die solves a lot of problems and headaches with average Remchester brass. And I can tell you I see very little to no accuracy improvement with neck turned brass and doing anything else to the cases with factory rifles.
