We've been able to walk up twice, but we are only a party of 2, and apparently that is much easier for them to accomodate. Last July, we had a late ADR, and they were able to take us about 2 hours early with just a 10-12 minute wait. In June the year before, they were able to take us for dinner without an ADR (there weren't any ADRs available when I tried to make them), but we did have to wait roughly 20-25 minutes for that one.
I'd try to make an ADR if you can, you can always cancel it later if you don't want to go, and that way you'll have it if you need it. There really is no reason not to make one, I don't see any stress with it unless it is a prepaid adr. If you do try to walk up, just understand that they might not be able to take you.