I think if you plan in advance the limited availability FP+ and ADRs you will be better off. Those types of things require you to have an idea which parks you will be in on which days, and think generally about where you will be in the park at the time of your FP+ and ADRs. If you get those basics covered, then you can go with the flow with the rest.
I would make sure you have the My Disney Experience App on your phone before you go (so you can book day of fast passes and book/changes ADRs, etc). Also get the Lines App for your phone.
From our experience, the Magic Kingdom fastpasses you really should get in advance are:
Seven Dwarfs Mine Train (60+ days out if possible)
the various princess meet and greets
Enchanted Tales with Belle
Big Thunder Mountain (I assume. It was closed while we were there).
The above list are also the rides to do at rope drop if you do not have fast passes.
We found that scheduling fast passes early in the day worked well for us. Once we used up our 3 pre-booked fast passes, we were able to get same day fast passes for lots of other rides (often immediately available). If you are going at a low crowd time, you may be able to go from fast pass to fast pass from afternoon on. We were often surprised by people waiting in standby lines when that ride was showing immediate fast pass availability and we were using a fast pass we had gotten 5 minutes earlier.
We found the following to be good same day fast pass opportunities:
Pirates of the Caribbean
Under the Sea – Journey of the Little Mermaid
Mad Tea Party
Winnie the Pooh
For Epcot, you have to pick which of the big three signature attractions you will get a fastpass for in advance (Frozen, Soarin, or Test Track), and should have an idea how best to minimize wait times for the remaining two attractions, assuming you want to do them all.
As for ADRs, if you have any must dos, then definitely get those in advance to ensure availability at the time you want, especially if they are at hard to get places, like Be Our Guest, Ohana, character meals, etc. I think advance reservations helps make your trip smoother since you can be assured that you have the time you want at a restaurant in or near the park you will be in. But if you are not picky about time or place, you can probably find lots of availability at the last minute, since people cancel reservations all the time. And there are good quick service options that may be preferable to you for the flexibility they offer (We liked Columbia Harbor House at MK, and Flametree Barbeque and the samosa truck at Animal Kingdom)
Another thing to think about in advance are show times. Hollywood Studios was difficult for us to plan since we wanted to sign up for Jedi training, and our itinerary was very heavy on shows that had set times. It would be good to at least have a rough sense of what is possible in advance.
If you are completely unfamiliar with some of the parks, then I’d recommend at least reading about the attractions and shows so you have an idea what interests you in advance.
I’d also recommend looking at park maps in advance, so you have an idea where things are. I found myself very confused by Epcot Future world, for example, and how to best cross over from one side of Future World to the other side. The layout of Hollywood studios also confused me. The MDE app does give you walking directions inside the park, so that is helpful.