It is not mandatory that you have them, but to be on the safe side, if you can set a time, why not? In 2008, we were all about making PS's and so the times for Goofy's were set ahead of time and we got seated within a couple of minutes of giving our name and paying at the front. In December 2009, on the weekend right before Christmas, we arrived at Goofy's without a PS/reservation, and the line of people waiting to give their names and pay at the front was very long. I thought we were in for a looooooong wait for a table. But as it turned out, once we gave my name and paid, we again only waited a couple of minutes and then were seated!! So we have had luck either way!