I don't agree with Notso.
Keeping the heels on the floor during takeoff is no good for two reasons:
1) Immediate braking is required, and having to raise your feet invariably delays this manoeuver;
2) Even more important is the fact that in a strong crosswind you are already applying pressure on the pedals: raising your feet on the brakes is impossible without releasing that pressure, thus for a moment you'll have to leave the rudder free floating!
Keep your feet on the pedals, making a conscious effort not to press the brakes.
Of course if you plan to rely on autobrakes only that's another story, but what if the autobrake disarm?
The only 100% safe technique is to use RTO plus full manual braking thereafter.
LEM