On my very first trip to Disney with my DH (then DBF), we were able to score a walk-up lunch to Liberty Tree Tavern no problem. However, I think that is the exception to the rule, and not the rule itself. We were also traveling in a relatively low crowd time, and it was a lunch we were looking to get, not an illusive dinner.
If you know you want to eat there, I'd stay on the side of caution and just reserve it. You can always cancel if you get to Disney and change your mind due to a schedule change or whatever.
That being said, sometimes I think people on DIS overstate the need for ADRs. On our last trip in June 2009, I was able to call dining almost every day and switched my existing ADRs around no problem, depending on our changing schedule for that day. I was able to score a last minute Narcoosees dinner, and a last minute Wave dinner on that trip...both of which turned out to be fabulous meals! We didn't need to book six months in advance either...only six hours!

I switched times around for a few other dinners as well, and it always seemed like the CM helping me had more than a few options available for every night. I wouldn't call that trip low-crowd levels either. It was in June, just as the kiddos were getting off of school and starting to flood the parks.