Ditto, Hairtrigger----- You only need to size shells that weren't fired in your gun, and then only if they won't go in your gun to begin with. I loaded many thousands of once-fired AA hulls back when I was shooting a lot that had been fired in other guns, and can't remember more than a few occasions of having a tight shell casing. They were all loaded on my old 650 MEC.
A chamber on the large side of tolerances may require shells be resized to fit your particular gun.