I do not want to argue with the experts; I am just an "amateur" gunsmith basically because I need to be my own gunsmith, but I never have a problem with an off-center cut opening a choke.
Perhaps just plain luck or, as you said, the chokes I opened have some parallel at the end of barrell (Winchester 101, Browning A-5, Ugartechea sxs, Remington 870 and some others I do not remember now).
What I noticed is that my 3 reamers (Expanding reamer made by Taylor and Jonnes, England) H6, H7 and H8 size, have some conical shape in the blades. If you measure them in the tip, you obtain about 0.005" less diameter than in the center or at the end. I think that this conical shape in the reamer helps to "autocenter" the blades and cut straight.
If you do this cutting by hand I can not see how you can cut off-center, because the reamer has the blades opposite to each other, so one blade can not cut if there is some "pressure" from opposite side.
I do not want to be a "hard head", but I still think an adjustable reamer is made to "autocenter" and made parallel cuts.
Anyway good luck with your shotgun´s choke opening proyect, you will enjoy to do it yourself and I am sure you will do in right!
Best,
