Likewise, I was taught to keep the the ball of my foot on the peg. While it does put the widest portion of your foot at the closest point to the ground, it provides you with better stability as you transition out of the turn and push your body back up over the bike. As well, when you have the middle of your foot on the peg, you have the peg centered on some of the muscles in your arch that you need to use, or in a position that doesn't allow you to maximize the force of your feet and lower legs. Mid-turn it isn's so much of an issue, but exiting will be easier and you won't have to take that extra 1/2 second to reposition your foot.
Was that an overblown answer to a simple question? Sorry.
