PS General Rule of Thumb
You probably dont need a PS unless your visit falls into one of the following catagories.
You want to do a character meal.
You are visiting during a school vacation or holiday.
You want to dine at one of the more popular restaurants (Cindys, WC, CG, Ohana, Narcoossees, Le Cellier, 50s Pt, Sci Fi)
You like dining at 7pm. Probably the busiest time for dinner.
Sorry to say that they dont save tables for onsite guest. But they do allow you to make a PS up to 90 days in advance.