Since speed for taxiing is slow it's not an issue, keep it up or down. For takeoff there is no recommended position. But in adverse conditions sliding the feet up for differential braking with full rudder in EFATO low speed reject can be a handful. In B747 classic rudders were not connected to nose wheel making very difficult. I always kept my feet on top and was very easy to slam the brake and rudder. In A320 with A/BRAKE and some control of nosewheel through rudders should not be difficult. As stated by Airbus as long as the objective to use full rudder deflection and differential braking can be achieved use any position you are comfortable with.