Here is my experience with PS. Note: the last two trips we have visited during early/mid August. Much to our surprize, the parks were not nearly what we'd imagined they would be like, having traveled at off times on other trips.
On both trips, I made both lunch and dinner PS's (1 per day, as others have suggested). Once, for dinner at Les Chefs, we arrived 45 min. prior to our seating and they sat us right away. Another, we had no PS and went to Italy, and they sat us right away. (we had also looked over the China menu and they were able to seat us fairly soon. (Both dinners were on the early side 5-ish).
Other lunch PS's, we ended up blowing off, since it just didn't work into the schedule at the moment. I might note, that the PS that we did not make at the MK, was where we ended up at Italy with no PS.
So, I guess, it all depends on when you are going, the time of day you are going, and how important is it to your trip experience to go to a certain restaurant that you want to try. I guess it also depends on your eating habits and if you can get by with just grazing. I also agree, that if a certain resort restaurant is desired, then I would make a PS for that.