I think it is a good way to start and get an idea of where you stand. If you are not saving any money when you put your plans in distripplanner, I do not think the dining plan is for you. If anything, it would probably overstate your savings.
For me, because we are doing the deluxe plan, the majority of our meals are characters meals, AYTCE, etc., the results were pretty close to my own spreadsheet.