I have travelled quite extensively in Germany at different times of the year and never found a problem in finding a Gasthof (B&B) for one night.
Is the same true for Tuscany in late June?
Tuscany is smaller with more tourists than you would expect. I would reserve in advance for late June.
We were fine in October without reservations. I'm not sure about June. We were driving so called each morning for a place we expected to be that night. We often had to call more than one place but never went homeless.
Arrive in the town you plan to stay in by say no later than 3 or 4. Your chances of getting a room are better then, especially on Fridays or the weekend. You can then spend the rest of the day sightseeing without worry. We've never had a problem doing it this way.
I think it's as much a matter of expectations and travel skills as it is time of year. It sounds like you have the experience from your travels in Germany.