Not looking to fuel a debate but simply adding more information/viewpoints for relavent discussion..
Another martial art that uses a light-heal foot technique is Fencing.
Beginners are commonly taught to completely raise heal of their back foot. As people progress they often get closer to the paper standard or even to just maintaining most of the weight on the ball. The front foot seems to take care of itself as you are typically leaning forward or backing up (which tends to put you on your toes). Granted, Fencing is brutally linear but wasted vertical motion (ie bobbing) is frowned upon and good fencers are usually very smooth and very fast.
As to Aikido, to execute a tenkan, if you are not already on the ball of the pivet foot you may already be too late.
As to the original question, I don't know about yoshinkan folks but a ballet instructor I have been fortunate enough to study stretch and bar work from would insist that you try to keep the weight centered across all the toes (neigher inside or outside edge)so that your ankle bears weight in proper alignment. This helps your knees too.
(to those of you who got to the bottom of my post..)
I am new here, to what appears to be a good place. I found you guys a couple of years ago when I started studying Aikido but I didn't stay as the terminology blew me away. I tried you out again last weekend and found that I only have to look things up occaisionally so I have signed on.