I did my reservations for a trip in May last week, and some of the restaurants I had to pick a slightly later time than I had planned because time slots were already taken. (no big deal though, only about half an hour) So, I say you should get in your reservations as soon as you can decide on restaurants, to be sure you get into the ones you want. You may be able to wait a bit longer if you have a small party, though. (we needed a table for six)