Yep, I asked about this not so long ago. As far as I see it, turning on the ball of your foot is the correct way to do it (for balance and speed). However this tends to twist your knee (look to see if your knee is pointing out to the side), and therefore could do long term damage.
The reason I first asked was because I now have bad knees - and now I use the heel of my foot to tenkan (when I remember!).