As you call in WDW with your day, just ask them what the earliest time would be for dinner.
It probably changes a bit with the Park's changing operating hours.
We have gotten one at 4 pm in the past & think it's the best time to go! This is before the big dinner rush & the restaurant usually stays pretty empty until 5 - that way you get lots of extra character interaction which is always great. We have a PS for 4 pm on our last day at the MK for our upcoming trip in Sept - have fun!
Due to the timing of our trip, we will be going to MK our first day. Knowing my family (esp. DH), they won't want to go back to the hotel for a break that first day. I was thinking that a nice early dinner at LTT woud refresh us.