It depends on how busy it is when you're going, and it never hurts to have a PS if you know where you want to dine. Having said that, we've had lunch at Kona Cafe three times and have never had a PS even though we visit at busy times of the year.
The first two times we only waited a few minutes for a table. The third time we had to wait maybe half an hour, but we wanted to look around the Poly anyway so we didn't mind. They gave us one of those flashing buzzers so that we would know when our table was ready.