The above answer is 100% correct.
On your last day, you MUST take the bus scheduled for you, or Disney isn't responsible for getting you back to the airport. They won't let you plan in advance to take a later bus, and you run a high risk of not being allowed onto a later bus if you just decide at the last minute to take a later bus.
In fact, there's no guarantee there even will be a later bus that you'd be able to use. DME buses don't do resort-to-airport runs on a continual basis ... they make pick-ups at the resorts ONLY if there is a reservation for a pick-up.
And even if there does happen to be a later bus that might work for you, the driver is under no obligation to let you on-board. In addition, the bus may be slated to be 100% full, and if the driver lets you on then you'd be taking the place of someone who belongs on that bus, if you know what I mean.
On the morning of the day before your departure day, you'll receive your scheduled DME time under your room door or hanging from the outside door knob of your resort room door.