My airline's B737NG FCTM:
During taxiing, the pilot's heels should be on the floor, sliding the feet up on the rudder pedals only when required to apply brakes to slow the taxi speed, or when manoeuvring in close quarters on the parking ramp.
The above refers only to taxiing, not takeoff or landing. I can find no reference to takeoff or landing. I therefore deduce that Boeing has no particular opinion on the subject.
When taxiing with tiller nose-wheel steering, it is easy to slide feet up to the brakes when needed, and I understand that riding the brakes during taxiing is not desirable - so that may be the reason for the policy.
However, I personally find it difficult to slide feet up the rudder pedals while holding rudder pedal pressure in a crosswind takeoff or landing. Therefore, I prefer to land and takeoff with feet up on the rudder pedals. Maybe my shoes have more tread than others. It takes a conscious effort to apply the brakes. I do not consider inadvertent brake application during takeoff or landing to be a risk.
I have never heard the topic come up before - this is how I fly and why. Can anyone on this thread actually quote a manufacturer's opinion on the subject?