I think some of it might depend on the time of year you are going? Also might depend what you are willing to settle for as far as resorts go?
If you think you'll know if you can stay the extra day about a week ahead of time, I'd call Disney to book it directly and book for the longer time. If you do it by phone, I believe you have 5 days to arrival to cancel any part of your reservation and it's no harm no foul (normally 6 for on-line.. so I'd swing 6 or 7 just to be sure). Otherwise if you don't, you're better off not canceling for a day as 1 night is what they charge you for the late notice.
If you can swing 6 days comfortably, I'd do that. Better to have the room and let it go, then not have the room when you need it and possibly worrying about not getting a room or having to switch to a resort that is very far from where you ended up!
If you won't know until you touch down on the property, then book for the less time. When you get there, ask if you are able to extend. If you wait until your last night to ask, depending on the time of year, it might be problematic.
