While I agree that there is probably little value in planning your whole day out hour by hour (like someone said, when does anything go entirely according to plan?), I also believe that a strict 'go with the flow' approach can be a recipe for disaster.
I think you definitely have to plan your dining reservations, which will also require you to plan in advance what parks you will be in on which days and at which times, and you should also factor into that plan various fireworks, parades, or special events (i.e. SpectroMagic isn't offered every day). Beyond that, I strongly believe there is also a need to plan to be at each park at opening, and have a game plan for the first couple of hours. That first two hours after opening can be a great time to get things done, if you have a plan. If you just show up and go with the flow and fail to hit certain popular attractions before the lines build, you may find yourself waiting in much longer lines later, or having late fastpass return times, or missing out an fastpasses altogether as some rides distribute all their fastpasses by early afternoon. After that first couple of hours in the park I think a go with the flow approach is workable.
Unfortunately, I know too many people who had a very casual approach to the theme parks, who didn't plan to be at the parks at opening, and didn't have any kind of plan on what to do when they got there......and they had a miserable time. If you are going during a very slow time maybe the casual approach will work out fine, but if you are going during a busy time you should invest a little planning time to go along with all the money you've invested in the vacation. You don't want to show up at the MK at 10:30 on a summer morning only to find that each ride you make your way to by going with the flow (Space, Splash, etc.) has a 90+ minute line you don't want to wait on, and before you know it you are hot and miserable and you've gotten nothing accomplished. I know people who hate WDW and will never go back because of this type of experience, an experience that easily could have been avoided with a little advance planning.