FAA Takeoff Safety Training Aid
I found some comments, based on objective data, in the FAA Takeoff Safety Training Aid. In summary, it appears not to matter which method is used.
The pilot’s foot position relative to the rudder pedal can also have an effect on the achievement of full brake pressure. It was noted during the Takeoff Safety Training Aid Human Performance Study that foot position during the takeoff roll tends to be an individual preference. Some pilots prefer to have their feet “up on the pedals� to be ready to apply full brakes if required. Pilots who prefer this technique also noted that their toes are “curled back� to avoid unwanted brake applications when applying rudder.
The other technique is to rest the heels on the floor during the takeoff roll, and then raise them to be on the pedal to apply full braking. No problems were noted with either technique One technique which did not work well was also noted, however. It was not possible to apply maximum brake pedal deflection, and hence full brake pressure, if the heel of the foot is left on the floor, unless the pilot has big feet.