I think it really depends on what time of year you're visiting and which restaurant you're trying for. It seems like Homecomin' and Boathouse are extremely booked so I doubt you'd be able to be a walkup there without a significant wait. Based on checking for my February trip, it seems like it wouldn't be an issue to be a walkup at many of the less popular locations, including Morimoto, Frontera, Raglan Road, Maria & Enzos, and Bongos. There are now so many restaurants at DS that it really disperses the crowds.