Depends on how you intend to eat and how much you want to pre-plan before your vacation. If you think your breakfasts and lunches will be at counter service places, then you won't have a problem.
However, just wandering up to a popular spot spur of the moment and expecting to get in right away would require a stroke of luck or a nice commitment of your time.
If you think you're inclined to a certain restaurant and a PS is offered, why not make the reservation in advance? You can always cancel. In fact, except for a few places that take your credit card for reservation to penalize a no-show, you can actually do the rude thing and just not show up.
On our last vacation, the rule that we used was when in doubt, make the PS. Although this kind of forces you to make a decision well in advance of which park you'll be hitting on which day, we found that our way around not deciding was to make a PS at two places for the same meal (like Coral Reef @ Epcot with another at Crystal Palace @ MK). Once we were at WDW and knew where we were heading for a particular day, we'd cancel the res we weren't going to use.
Disney didn't seem to cross-check our reservations and we double-booked twice. I think as long as you don't get excessive, like 20 PS rezzies for a 4 day trip, they don't mind.