In March, we decided to go mid-June. We were already within the 90 day window. I probably started calling around day 75ish. We have Crystal Palace for lunch (I actually had it booked on two different days but did cancel one), 'Ohana (dinner), Akershus, Whispering Canyon (dinner), Hoop-dee-doo and a few others that aren't so popular. I am still hoping for either a CRT or Park Fare cancellation, but I'd say that overall, even booking within the window, I was able to get a pretty good selection.
I didn't try for places like LeCellier, California Grill, Coral Reef, so I don't know if I would have been successful for those.