I wouldn't leave them alone in Paris longer than necessary, but along the Loire a Velo is mostly small village and tourists. Even in the cities, we never had a second thought leaving them locked on a rack with the panniers still on them (as you mentioned take the bag with valuables). Most of the hotels had secured parking for bikes.
I used a combo of a cable lock and a U-lock. The cable lock was probably enough for short sightseeing, but I had the U-lock for overnight stays anyhow.