As far as I can see the main reason for designing in a longitudinal dihedral would be to achieve a cruise set-up with the wing at alpha for best TAS/Drag...
While that may be the objective, it's often missed by a mile. Ever notice the deck angle at cruise?
The fuselage is a pretty inefficient lifting body, and yet at typical cruise Mach, it's at maybe 1 or 2 degrees noseup. Shouldn't it be level? I think it's because the aero designer optimizes the wing incidence for unrealistically high Mach, and then misses that calculation on the low side. So, in order to cruise at real world conditions (crude oil approaching $60/bbl), the galley carts roll aft.
The DC-10 could definitely have benefitted from another 2 degrees incidence.