We were instructed to keep heels on the floor during take-off, (Airbus FBW
There is no official position on this by airbus. Both feet up or down is left to personal preference. However with heels on ground in case of engine failure at low speed on wet runway with unfavorable cross wind it takes some doing to apply full rudder and differential braking especially on aircraft like B747 classic where nose wheel was not connected to rudders. Feet up during takeoff roll requires a flat pressure on rudders more towards heels. It's not difficult. I have done it all my life.