i would say it depends on the time of year you are going(super busy vs not too busy), the time you would like to dine(early or late), and of course the popularity of the restaurant. we make PS for all our sit-downs no matter when we go, the time we eat(usually early) and popularity of the restaurant.
It is simple just figure you always need it unless you like waiting a long time. You will what some time even WITH priority seating.
If you make PS and don't need it that is not a problem unless you have booked a character meal , show meal, or on a holiday when they take a credit card deposit otherwise does not matter if you show up or not but if you do at least you are on the list.