Deciding whether to prebook every intercity ticket or preserve flexibility

We'll move between several cities by train during a two-week trip. Booking every ticket now would secure seats but make each day feel rigid; buying on arrival would keep us free to linger but might cost more or leave poor departure times. I'm planning to reserve the longest, busiest legs and leave shorter regional journeys flexible. How do you choose which transfers deserve a firm booking and which are safe to improvise?