You will have to prepare to pay higher prices at times since last minute rooms usually cost more. Of course, you will have to be flexible as well as you never know whether a hotel near your stop is available when you get there.

I have followed this strategy in the past driving in Europe. I did run into issues at times though. One night, when we were trying to make a stop in Rennes, France. All hotels were in the city were sold out due to an event. We had to drive an extra hour in the dark to find a room. Then on the same trip at Mont St-Michel, we got the last room of a pretty decent hotel but the room was near the kitchen area. We could smell the food preparation the next morning. In Spain near Jerez/Cadiz. we called prior to dinner and the hotel had one room left. When we got there after dinner, the room was gone so again, we had to go elsewhere.

These days though, you can just use one of the hotel search sites in the afternoon when you should have a pretty good idea of where you want to stay. I suggest you make the booking to avoid the sort of experience I described above.