Breakfast tends to be pretty empty so you probably wouldn't need one. For dinner, like PP said, if you are getting there right when they open you should be okay. Also you'd likely be okay after normal dinner time, like 8pm. From 5-7pm it can be pretty busy so if that is the time you'd like to eat you might want to make an ADR. I have also found that there are often same day ADRs so even if you don't decide until that afternoon, give disney dining a call and make that ADR.