I've been thinking about this too as I plan my first CC tour. While one of my thoughts is to make a fairly last-minute go/no-go decision based on weather, I do think I want to book at the time I make the decision for areas where there are few alternatives.
In your itinerary there aren't a lot of choices - particularly for example Brunswick I think there may be really only one place to stay, and with Harper's Ferry currently unreachable from the trail it may get more pressure than usual so you might want to book that one in advance.
It's the people camping who have more flexibility but I just don't see carrying that much gear on my frame.