I try to have a plan for the next year's vacations a year in advance. (I'm kicking around ideas for next year now, 3 weeks before I actually start this year's trips.) Drives my family bonkers but with two timeshare systems and various work schedules, I need to have some handle on what we need.
That said, I have added days and futzed with the schedule as the date drew closer. I find my family tends to remember their schedules at the last minute.
The thing that worries me most is trying to snag accessible accommodations. The longer I wait the less likelihood Disney will have the room size I want. (I had to book a 1bedroom this April instead of my planned studio.)
With Disney, I figure as long as I cancel before my banking window closes, I'm golden. I always try to carry over points so I'm dealing with a surplus. I don't want to suffer distressed borrowed points.
I find once I've made the travel plans for the year, we seldom cancel. In fact the only trip we ever cancelled was a last minute medical emergency and in that case we got our timeshare week back. It was with Marriott.