Generally, we full length size to maintain smooth bolt operation, and this is required because loads that take advantage of the full potential of a given caliber expand cases to the point where that is needed. The trick is having a FL die that just barely moves the brass so that a good fit in the chamber is maintained, and brass life is not shortened by overworking. Perhaps you could tell us more about your rifle?
I am using a Remington 700 action in 308 that has had work on the chamber and new barrel
As far as setting a neck die is concerned, there is no need to back them off, because they are not built to push back the shoulder, or size the body.
I have not started using the neck dies yet. I was contemplating there usage after fire forming.
Generally, my least favorite kind of dies (based on the concentricity that they produce with unturned brass) are one piece neck dies, followed closely by one piece FL dies. Luckily, there are better alternatives. The one kind of die that may require backing off from the shell holder, by more than a quarter turn, is a conventional seating die that has a built in provision for crimping. I generally set these by screwing them down on a sized case that is in the raised position in the press, where it would be with a bullet fully seated, and carefully noting where I first start to feel the case, and then backing the die off perhaps a quarter turn from there, to make sure that there will be no crimp.