I can only tell you this: in each state, its a traffic law violation to coast down a hill with the transmission in neutral.
Based on this law, i would say that keeping the cars transmission in D, would be mandatory. i have nothing to backup my opinion, other than this other law.
I worked a traffic accident where the car in front of another car was waiting for the light to turn green. both cars were on a small incline. front car was in neutral(manual transmission) and waiting for the light to turn green. foot slipped off the brake pedal and rolled backwards into the behind vehicle. front vehicle was at-fault for not having his vehicle under control and improper backing.
So you see, it can go either way, depending on the circumstances.