We usually make our dinner reservations for around 8-9pm. That way if we decide we want to eat somewhere else earlier and can get in, great, then we just cancel our first ADR's. If not, then we know we always have our late reservations.
Also, by doing them late it doesn't interfere with our plans. For instance if you are at a park and everyone is getting tired at 3pm, then you don't have to force everyone to stay until 6pm, just because you have reservations. You can go to your room, rest, swim & freshen up and then head back around 7pm.
Also, at least from my observation, most of the buffet food seems fresher after the dinner rush when it has been replenished. Also, we seem to get far better service than during the busy 5-7 dinner hour rush.