I would definitely plan to some degree......depending on when you go you may NEED ADRs for table seating at restaurants......otherwise you may have to wait a long time, be turned away or just have to do counter service, and there are too many reasons to NOT do counter service ALL the time......table service is good for breaks, better food, relaxing,etc.
Also, it's good to have an idea of what's at each park that you MUST do, the 5 things or so that each member of your party REALLY wants to do, and be sure to get them done, check times for shows, see when the best time is to visit certain attractions, get fastpasses for them etc. THat is info you'll find at Tour Guide Mike, on these boards, or in various guide books you can borrow from the library or purchase.
If you just "wing it" you risk not knowing what's there, missing key attractions, or doing things in the "wrong" order, and having to wait longer than you would have if you planned it out more.
All that being said, it's good to be flexible as well, we didn't do some ADRs we had planned (just remember to cancel if you plan to not show up), and missed a couple of shows and rides we had wanted to do as we decided swimming at the hotel was a better plan for that REALLY hot first full day there!
Good luck!