If you plan on doing any Character breakfasts or lunches, then yes, you do need ADRs. For the really popular restaurants (Sci Fi, 50's PT, Rainforest Cafe, Le Cellier, etc), I'd play it safe and do ADRs as well. The resort restaurants that aren't Character meals, I would feel not so worried about.
If you don't want to do ADRs at all, plan to go really early or really late- not peak meal times. You should have a better chance of being seated then.