I like 180 days (180 + 10 for resort guests.)
For most people going to Disney, the vacation is not a last minute decision...due to the expense & logistics, a lot of people plan these vacations well in advance, so what's the big deal figuring out details like meal plans in advance too? You pick your resort, don't you? Why would you not plan the meals out too (at least the table service meals.)
As far as whether it should be 90 or 180 days, there's not a whole lot of difference between planning 6 mos. out or planning 3 mos. out. Neither is last minute, fly-by-the-seat, winging it...neither option is "spontaneous" at all. Personally, I enjoy researching restaurants, and planning it all out. And I'd really hate to spend all that money on a Disney vacation, & then find out I can't get into any of the places I want to eat when I want to eat.
If you want to be totally flexible & unscheduled, then just show up & see what's available day-by-day. You may have a wonderful time. But if you have specific places you want to eat, and want to choose what time you'll be eating there, then book in advance.