In general, the first buses are leaving the resorts starting about 45 minutes prior to official park opening time. Sometimes a bit earlier as some buses hit the roads earlier than others, or they have shorter routes. This morning, for example, the first MK bus left All Stars at 8:10 for a 9:00 park opening. The first buses will indeed get you to the park before opening time.
You don't gain much by getting to MK earlier anyway, since Main Street is no longer open early for shoppers. They start letting people through the turnstiles at about 8:45 for a 9:00 rope drop at the hub in front of Cindy's castle. In the weeks to come, during peak holiday season, they may open the turnstiles a little earlier to avoid crowding in the plaza in front of the turnstiles.