Is it worth noting (or is it even true?) that any aircraft with anhedral is also a high wing aircraft?... I suspect the degree of lateral stability gained by having the fuselage essentially hanging below the wing enables some degree of anhedral to bring the net effect to something closer to neutral... (though undoubtedly still not neutral). But in some of the big cargo birds, perhaps without the anhedral, rolling the things would be quite a chore!
Just a thought....