It's best if you do...that way you can hopefuly get the day and time your wanting. But if you'd rather just wait and see (play it by ear)...you can always just take your chances and check on the day you are wanting to go.
I'm not sure how many cancled reservations they get...or if some times are more highly sought...but the night we went for dinner and the day we went for brunch...the place was rather empty. After hearing the need to go early and get the Palo rese first thing...I expected the place to be packed....but it was FAR from full both times.