Agree with other posters. It all depends upon the time of day and what restaurant it is. There have been several times where we've checked in and seen folks come in w/o reservations and are turned away. If you try going at peak lunch hours, there's no chance unless it's a fluke thing.
Go ahead and make your ADRs just to have them. When you're actually there, if you change your mind just get online or call. We've had pretty good luck over the years making last minute or even same day changes - because people will cancel at the last minute as well.