Not putting in your park brake on at at any time while parking is inappropriate. Not engaging it even while on a small incline/decline will ultimately hurt your tranny.

The sequence for stopping and parking your auto tranny would be as follows:

Come to a stop, keep foot on brake

Put shifter in "P" while STAYING on your foor brake

Now engage park brake, release foot brake. Reverse this procedure when leaving with your car. There is reason for why you can not move the tranny shifter until you get on the foot brake first.