For me it's all heel down, any "Deathmetal" type of speeds, and I go into either "Heel-Toe" mode, or I can even do the Derek Roddy style of playing - which is essentially heel-toe, but only your toe is on the pedal, but since there is maximum spring tension on the pedal, you can't play it at the back of the pedal..... you have to do it up near the toe end of the pedal.
You know how, when you were a kid you would sit at the dinner table, and do that thing with your feet where you would sort of dig the toe/ball of your foot into the floor, and bounce your leg under high -tension? That is exactly how you play the Derek Roddy style of playing for speed.
Tim