Like others have said, I think it's a personal decision on how much planning is enough or too much for yourself and your situation. I recommend to all of my clients and friends that they at least plan out their ADRs, what park they want to visit each day, and have a rough idea of what attractions are "must see" for their party. That's the bare minimum. You wouldn't travel to New York City without at least that much of a plan, why would you go to Disney so unprepared?
As for spreadsheets, binders, etc, my personality loves them. For me, planning the vacation is part of the fun. Some see detailed plans as marching orders, and if you're not careful, they can be. But they don't have to be if you remember to remain flexible.
Here's my story. Back in 2007, my parents asked me to help plan for Disney trip for our family. I went the detailed route - spread sheets, binder, step by step touring plans. etc. My father when he saw all the work that went into it thought it was ridiculous, and looked at it as marching orders. "Are you going to schedule in our bathroom breaks?" he would ask mockingly. But once we were on the trip, he realized the benefit for our family of 5 (and all rather opinionated and stubborn, mind you). There was no argument of what to do next or where to go, as we had a step by step plan. Did we deviate from the plan at multiple points? Yup. Never did we "have to" stick to the plan, but it was nice being there to help us keep moving and not miss anything important to us. At the end of the trip, my father was all praise for the amount of work I put into it, because he realized we saw so much more with the plan than we would have without it.
