I have 2 smoothbore long guns, one in the Tule F.d.C. style. I can shoot okay without the rear sight at the range in normal positions with good form, but things change a bit in the field with some shots in awkward positions, it changes the relation of your eye (the rear sight) to the rest of the gun and front sight. I definitely prefer having the rear sight.
Type G British trade guns had a rear sight, so a smoothbore with a rear sight during colonial times isn't unheard of. No documented examples of Tule guns having them that I'm aware of, but I'm not an expert.
I've seriously considered putting a very small unobtrusive rear sight on my Centermark Tule F.d.C.