Bench rest shooters try to keep the neck dimensions within .002 in of the internal diameter of the chamber on their rifles.
The neck area dimension is usually cut to a diameter that is to tight to accept factory brass, as it comes, with a bullet in place.
They turn down the necks, so that there is .002 clearance with a bullet inserted and the chamber.
Some hunters, that are serious accuracy freaks. get special reamers made up to bring the clearances down to minimum, to eliminate another variable.
I have a 338-08 with such a chamber. It was cut that way because I used a normal 308 Palma Match reamer to cut the chamber, then opened up the neck throat with a special neck/throat reamer. When I bought that neck/throat reamer, second hand, I didn't realize it was cut to minimum specs.
I was using Norma Match brass, which may or may not have had necks thicker than most commercial brass. I had to turn all of the case necks so they would fit into the chamber of my rifle.
I'm lucky and have a lathe. All I need to do is turn down an appropriate mandrel, slip the neck over it, while holding the base in a holder on the tailstock and turn it to the appropriate diameter.
For those that don't have a lathe, Brownelle's makes a handy little hand tool that does a reasonable job.
The only reason I can see for turning neck diameters on any cartridge is to get uniform neck tensions.
Most rifles have rather generous necks, to accommodate any commercial load. When you remove material, the necks have to expand more and as such get worked more when resizing. Likely not enough to make a lot of difference.
