"The problem with selecting a place in advance is that you end up passing by lots of places that are perfectly fine while you spend time looking for a particular address."
I can't speak for all countries in Europe, but in the countries with which I have experience (Germany, Austria), that's not true. Virtually every town has a website with a list of accommodation, which I have found to be very complete (particularly compared to sites like booking.com or Tripadvisor). I find it far easier scour these lists for just the right combination of facilities, location, and price, than to use the TI office or walk around aimlessly.
A few years ago, I experimented with the "no reservations" idea. In one town, TI got me a very nice hotel within short walking distance of the TI office at a reasonable, but possibly not optimised, price. I would have preferred a Privatzimmer, but they said that was what was available.
In the next town, I arrived a day early, and instead of calling ahead to the Pension where I had a reservation for the following night, I just arrived, went to the TI office, and got a recommendation. The hotel was a little more expensive and a little farther away than where I stayed the next night.
This was in March. For my last two trips, in November 08 and last August, in a least one town each trip, I had to write half a dozen emails, at least, before I found a vacancy, and this was a month or two in advance.
So, in my experience, if you wing it, you will probably find something. It just might not be what you might have wanted had you had everything to select from in advance.