Noon is always given if you call the front desk and ask for an extended checkout. And from there is just depends on how busy the hotel is. You'll probably get 1pm with the Lowes First membership.
Late flights home can be tuff because the airlines back up. Our family once spent the night in ATL because our connecting flight arrived late and our plane had already left. We literally slept at the gate thru the night before boarding a flight home at 8am.
I usually "feel" like the trip is over on the last day. Sure, we usually go down and visit the pool, or a park for a few hours, but that's about it. I'd suggest you book your flight home at 4-5pm, but I wouldn't go past that.
What the late check out does, is let you visit a park in the morning, come back to the room to clean up, and then head out for the airport. It means you can take your time and relax on the last day. But I wouldn't push it into the night, or you may get stuck if your plane is late.