I would detect the whole thing including the ditch (if any) surrounding it. I would imagine people walking or riding the trail would lose items in the middle and also pitching things off to the side like they do with the modern beer cans. Maybe even take a look at topic maps and focus on the lowest elevation side for things that might have washed out in the direction of water flow.
I have permission to detect an old road used by waggons and buggies in the 1880s .never yet detected. Should I detect the road itself or a few yards on either side?or somewhere else?
IMHO: It is a waste of time to detect randomly along old roads. Instead, your time is better spent figuring out where they stopped, camped, rested, etc.... Eg.: stage stops, etc.... THAT is where the fumble-fingers activity would have been occurring.