We went during free dining last year and we walked up to Chefs de France and got a table (I think it was around 5 or 6 oclock).
We had an early dinner reservation for Le Cellier and a man walked in to see if he could get a table for him and his family (I can't remember how many were in their party...maybe 4). And I snickered under my breath and thought how he must be a Disney newbie and they said yes!!! He ran out and got his family and they got a table at Le Cellier!! (serves me right for being a snot).
I don't know how usual it is. When we went to Mama Melrose for our Fantasmic package dinner the whole entrance way was PACKED, I doubt they were taking walk ups, same thing with WCC the night we ate there. I wanted to get 'Ohana dinner for that trip and wasn't able but we didn't try walking up so I don't know about that one.
We did call about two or three days before our trip to book a signature restaurant and they had availability at Yachtsman, Narcoossees, and Citricos. We chose Yachtsman because the other two reservations were for late (I think 8.30 or maybe even after 9).