I used to use a 6:00 hold until it was pointed out to me that some light conditions can make finding the bottom of the bull uniformly a real challenge, particularly if the target is getting fuzzy due to older eyes. The center of the target is easier to find, particulary if your front blade appears to be the same width as the 100, 200, or 300 yard bull, assuming you use a round bull. If you see the bull inside the width of the front sight blade you're not centered. Switching to a center hold has improved my scores dramatically.