I think it depends on when you're going, how many are in your party, and where you are likely to eat. If there is only a party of two, and you are willing to be flexible with your dining times (eating meals earlier or later than the normal lunch or dinner times), and you're willing to be flexible with where you eat, you should be able to do some table service meals without an ADR. DH and I were in WDW Christmas week and had a late dinner at Kona without a reservation, and were seated at Sci-Fi diner for an early lunch without a reservation (and we got a car, too -- LOL!). So, it can be done, but just don't count on it. So, if you don't get in somewhere, just move on and don't be disappointed.
That said, I would make at least a few table restaurant ADRs just to know you have some. You can always cancel them if you decide you're not going to use them, but I like the peace of mind in knowing I have a couple ADRs just in case. And if you are more than just two people, or if you have children, I would definitely make ADRs because it's much harder to seat larger parties and kids usually do better if they're eating at known times. Have a wonderful trip!
-Dorothy (LadyZolt)