Hi and welcome to the DIS!
A lot depends on when you are going. If you are going at a peak time you will have trouble finding a seat at a TS restaurant without ADRs. If you are going in off season, you could walk in to some restaurants.
There are a number of restaurants where you will likely need ADRs no matter when you go, like CRT, Crystal Palace, Chef Mickey's , Sci-Fi, and Prime Time.
Also, if there is any place you have your heart set on eating, make an ADR. They're free and you can always cancel later!
Bob