Paul, were not talking about taking alot of brass off normally from the outside rim to the primer pocket it is more oval from the stamping. Rim thickness varies if you care to measure it and primer pocket depth also varies myself I use a primer pocket uniformer that cleans and cuts the depth and I think most shooters use one. If the thickness of the rim was uniform you would need to worry about primer pocket depth if buying a bag of Win 223 brass.
The shell holder for the Wilson trimmer simulates a rifle chamber and it sits in a rail system that gives a square cut so if you square the case mouth why not square the case head alittle since that thickness varies? For my match brass I do alittle extra with it since we aren't talking about alot of cases. I would also assume web thickness varies and that was one of the problems when the old Sako 220 Russian case dried up and alot tried to make them 6ppc cases couldn't get the web right.
Paul, I'm 64 and never stop learning. I knew guys that even rebate the 6br rim to fit the 6ppc bolt alot was going on back in those days most would wonder about.
Sinclair sell a primer pocket tool that cost arm and leg, they also include a shim pack since rim thickness is different for a 22-250 to a 308 those shim sizes are .005",.010",.015" and .020" since that tool locks the rim against the shell holder.