It is the same as normal heal toe, what ever side of the foot board feels better is the only question. I like the innter of the foot board so it is closer to my snare. Some people say that it is easier to do it off the foot board. I agree, the only problem is that it takes longer to adjust to that possision.
Heel-toe is a misleading name for the technique in the first place. It is not your actual heel that produces the first stroke - it's the motion of dropping your foot almost flat against the board, then pivoting up onto your toes for a second stroke. The stroke is produced from the middle of your foot, more from the ball than your actual heel.
Anybody with any size foot can do heel-toe - this has been said at least a million times.