For a 9am park opening, MK lets people in the front gate in the 7:45-8am range, roughly speaking. Guests are free to roam Main Street and the Hub. Many shops are open, Starbucks is open. PhotoPass photographers are usually out, as our Main Street vehicles.
The opening show is on the castle stage at approx 8:55am, after which the “ropes drop” at the various spokes off the Hub and the park is open.
Given the nearly hour of “loading time” before the park opens, you have a lot of flexibility of just when to arrive. You can get there early and enjoy the sights/sounds of Main Street, or get there a little later in the hour - it’s really personal preference.
The only real time sensitive thing would be if you want to head to 7DMT. If that is the goal, you’ll want to be near the spoke that goes to Tea Cups. The crowd in front of that spoke will grow more quickly during the pre-9am time than other spokes.
Have fun!