I'd recommend you generally book ahead if you can. That gives you the most options to pick a place you like, and know that it's reserved. I booked ahead when I stayed in Monterey
and Carmel By the Sea
, and you'd probably want to book something in Morro Bay
on the way down.
I used to use HotelTonight
to book last minute hotels on the same day, but in general I'd probably only book on the same day if it's a bigger city where there are lots of hotels. If you're doing the coastal drive, I think you should book ahead to plan your trip.