We scored a walk up about 3 years ago (October) - it was a very late lunch - I think around 2.30pm - never hurts to try but an ADR is definitely advisable for this one.
About three years ago we had an ADR. And the people in front of us at the podium did not. They were seated. And I checked in. I commented to the hostess that the folks in front of us were lucky. She said it was the first walk up they had been able to seat for over a week, and that was the only one they'd be able to seat that evening - unless something happened - like it started raining hard.