Staying at Port Orleans - Riverside, I would be at the bus stop by about 7:45 for an opening time of 9:00 at the Magic Kingdom. The early time is to be sure of arriving at the park before opening time. Since the bus normally stops first at the French Quarter, and then to the stops at Riverside, there is a chance that the bus will be full and not stop at your closest bus stop.
Looking at a map of Riverside I see four bus stops; one at the main building (lobby), and three others around the property - "west," "north," and "east." The bus stops at the main building first, so if you are near that location, or want to walk over there, you will be sure to board the first bus that comes along, and probably get a seat as well.
Enjoy.