Personally, I don't understand this. Seems to me that it's just a matter of personal preference. I center the pin in the peep. To me, and it's just my opinion, taking the time to center the pin guard in the peep and then putting the appropriate pin on target is just adding an extra step. Before I draw, I'm going to know which pin to use. I don't see adding an extra step into the process.
As to using proper form, I can't see any difference in centering the pin and centering the ring. You will still have to move your bow arm the required amount to get on target regardless of what you center.