Airbus doesn't specify as far as I'm aware, but talking to some of our training captain, I shifted from heels on the floor to heels up. The risk of brake application is minimal during TO if you keep your toes up, but needing to apply differential braking at max crosswind could be difficult otherwise. The time taken to shift heels up to the pedals in a heavy crosswind might be just the thing that puts you off the side of the runway. Same with max manual braking if needed.