I usually like to travel without reservations but, in the case of the coast highway, reservations are probably a good idea. But it is really hit and miss. We have done trips during busy summer times on the coast where lodging is easy to find and, at other times, we can't find a room or a campsite to save our life. We had one memorable night in a horrid, fleabag when we couldn't even find a spot to pitch a tent, much less a better hotel room. It was late, we were zonked, and we ended up bringing in our sleeping bags and laying them on top of the mattress because we just didn't trust that things were clean enough for us...and used our own pillows as well. Yucky place.

You might get a good guidebook to hotels along the coast (AAA is a good resource for this) and wing it a bit. In other words, hit the road without reservations but each day, when you get a feel for where you're going to end up, start calling ahead via cellhone (if that's an option for you) and make a reservation that day. This way you have some flexibility but, at the same time, don't end up dog-tired without a place to hang your hat for the night. Of course, you're still taking your chances this way and may end up driving farther than you want before you find lodging.

Another option: make reservations but make sure you have all the details of their cancellation policies. Then you can actually make more reservations than you know you will need and just cancel the ones that you won't need along the way prior to any charges kicking in.