In summary:
You still need a park reservation to visit any park.
Unless it is after 2PM, then you can tap in anywhere without a reservation.
But if it is a weekend and you want to go the Magic Kingdom after 2PM, you still need to make a park reservation or you have to make a reservation and tap in at another park, before being able to hop to the Magic Kingdom.
However, if it is a "good to go" day, none of the above applies.
Unless your particular Annual Pass is blocked out for the good to go day, in which case you will not be good to go.
It's SO Easy!