As ShellyF said, it depends on the time you land, and how long a drive you have to get home. I like to arrive at my home airport by 9-9:30, so that means no flights after 6:30pm.
Another consideration I have is how long I've been in WDW. If I've been there more than 5 days, I usually try to get a midafternoon flight home..that gets me picked up around 12-1, which means I can sleep a bit and get a good meal in before heading to the airport.
If I really want to spend a bit more time at WDW, I take a later flight..usually around 6 or so. That gets me picked up by 3ish, so I can go to a park for the morning, and have lunch, then back to my resort by 2.
I check my bags with RAC by 8am, no matter when I'm leaving that day. Then, if I'm leaving after checkout time, I leave my carryon bags with Bell Services and get them back when I return to get the bus..but be sure to allow time to get those bags. Sometimes it takes them 15 mins or so if they're busy.