There was once a 1930's high-wing private ship with no dihedral - but the lines of the aft fuselage made it
appear that the the wings drooped. The designer felt this was a marketing disadvantage, so in a later version, he added about a 1/2 degree dihedral to "correct" this illusion.
(Yes, I got this straight from the designer's mouth in a little Chinese restaurant in California 36 years ago.)
