It really depends on what time you want to eat, the time of year of your trip, the size of your party, and what place you want to eat at.
If there is any place you are dying to try, then by all means make a PS.
We've only been turned away twice so far due to PS all booked....once at Le Cellier and at the Garden Grill, it was a Friday night.
DH and I make very few PS, but we're just two and eat dinner really late and don't travel peak season. Of course our next trip, we maybe making a few PSs ahead of time because it's a busier time.
I hate being tied down to many PS's on vacation.