I center the pin, however centering the sight is a relatively new way that is catching on and I can see how it would be more accurate.

I use the pin since I have a rectangular pin guard.

Whichever you do you will shoot fine, as long as you do it the same everytime. They eye wants to center things, so its all in what you are centering, the pin or the ring.

Centering the ring will also be more or less effective depending on the size of the peep opening and the sight ring.

--Bob