Currently, if you are entering a WDW park using an AP, you will need a park reservation for your first park of the day, every day. It doesn't matter if you're onsite or not. There are a few exceptions, like if Disney declares a good-to-go day (usually because of lower crowds expected that day. You also can enter the parks after 2PM without a park res, except (I believe), MK on weekends. If you are staying onsite, you can make park reservations for the length of your stay, including check-in and check-out days.