I think it's easy to travel without reservations. There are certain caveats; If you're going somewhere you know is going to be crowded, you need to be realistic about getting lodging. Book ahead there or have realistic options (like travel to the next town by train and find something there).
My last big trip without reservations was to Spain in April, high season in Spain, 2012. I created a list of places that would be decent lodging choices for all of my expected stops, put them on a spreadsheet, and kept them on an Ipod touch. Each travel day, I'd stop around ten and call ahead for the next destination.
In everywhere but Granada, my first choice was available. In Granada 1st choice was "completo", full. 2nd choice couldn't give me a room for all the nights but promised to figure something out, but I might have to change rooms. 3rd choice was available (but we decided it wasn't that nice of place). 4th choice was available and was perfect for us. Arriving in Granada without reservations to the Alhambra was also trying, but we used the advice in Rick's book and ended up getting in at a good time.
The only time I actually thought I was going to go homeless was on opening day of Oktoberfest in Munich. I made about ten calls with no luck, tried the TI but it was a zoo of people trying to find a place, went back to the phone with a longer list and eventually found a nice pension at a decent price - so even then, I ended up okay. When I left Munich, I had no problem finding something in Salzburg with no reservations.