Considering WDW is almost booked solid for July, I would suggest that you make any ADR's that you want right now and not wait. I was there at the end of April, Epcot was as empty as I have ever seen it, and Biergarten was turning people away at the door! Dining has become extremely hard to do as a walk-in. Now, I will say that for Sci-Fi, you probably won't have a problem. But, if you know you want to eat there and on what day, approximate time - make the ADR now and avoid disappointment and a long wait.