The round pin guard was developed as a way to ensure correct peep/sight alignment. The correct way to aim is to center the round housing of your sight with the round peep and then move both as one unit until your pin is on the target. Centering you pin in the peep will also work, but you are aligning a much smaller object within the circle of the peep.
I'm sure there are several ways to put the pin on the target. Some will tell you to stand straight up and down and lower or raise your arm, others will say to keep your arm straight out to your side and bend at the waist. I say do whatever is the most comfortable and relaxing.
Pathfinder Arrow Wraps