I have tried both methods on takeoff in various aircraft types. In general, I feel more comfortable with heels on the floor because I can make finer adjustments to rudder deflection.
The main risk being discussed seems to be associated with a slight delay in braking. On the B737, there is a two second period allowed to establish the stopping configuration (3 seconds on more recently certified aircraft like the A320). IMHO, the 'heels on floor' method is low risk because a number of events would need to coincide before it was a problem, including:
1. The autobrakes would have to fail.
2. I would have to be relatively slow in applying maximum braking.
3. A failure would have to occur very close to V1.
4. The TORA would have to be a balanced field or close to it.
While this is not inconceivable, I am not sure if the risks of dragging a brake on takeoff are outweighed by the risks of 'heels on floor', particularly as I rarely see a balanced field (normally atleast 1000' to spare).
All that aside, if there is some objective data available I am keen to see it.