Not sure you really need to, with just a few exceptions. But we've made breakfast ressies every time...and the only place it really would have made a difference was Chef Mickey's. When we went at 7:30 there were empty tables, but by 8:00 a.m. they were wayyyyy backed up and lots of people waiting. I've read that 1900 Park Fare & Cape May also get backed up, but can't say from personal experience.
Everywhere else, including Boma (9 a.m. on a Friday) , Crystal Palace (8:05) before park opening, Hollywood & Vine (8:05) before park opening, and Kona Cafe (7:45) all had many, many empty tables...as much as 75% empty.