I have asked a related question on finding accommodation during road-trip in the Czech Republic. One of the given answers suggests to inquire at the local tourist information centres. Given that I will be traveling during the summer holiday, how reliable is it rely on tourist information centres in the Czech Republic? I know that this approach doesn't work in the south of France, where you really need to book ahead. How is this in the Czech Republic (not considering Prague).
The answer depends on the city. In Prague there are 1000+ hotels, not to mentions the dozens of properties on AirBnb. While it's common to see 90+% booking rates on Booking.com, there will always be some hotel with free rooms at a reasonable price. No need to go to tourist information centers, just book online.
In a place like Cesky Krumlov the selection of hotels is very limited. The local tourist information center might help, as there are many hotels in the country-side without proper online booking. However even in the worst case scenario you can always go to a nearby city - in the case of Cesky Krumlov I've simply booked a nice hotel at nearby Ceske Budejovice.
Therefore my advice is not to bother with pre-booking (unless you really care about staying in city centers) and simply finding the closest free hotel on the spot.