without stirring up the pot, the optimal pedal stroke (from an energy efficiency and power output pov) is neither toe down nor heel down. there is a slight ankling movement that happens throughout the pedal stroke. however, I understand that most of you are referring to an extreme toe down technique (what I refer to as bike ballet).

toe pedalling engages the smaller muscles in your lower leg as opposed to the larger muscles in your upper leg (and smaller = less force produced). in addition, and extreme toe-down technique will place stress on your achilles tendon which can lead to an over-use injury.

one of the most common reasons I see cyclists toe pedalling is that their saddle position is too high, causing them to over-reach at the bottom of their pedal stroke.

yes, it can take some time to develop the most efficient pedal stroke, but it's time well-invested in the long-run.

hope this helps!