Sorry I don't have a reference, but as long as you are outside of an object, its gravitational force on you is the same as if it were a point-mass (at least for classical physics).
As soon as your test object is non-point you get tidal forces due to the field typically being nonuniform. As soon as your object is not spherically symmetric you get a field that looks nothing like that of a point mass.
I'd guess this is what the OP was recalling.
That doesn't apply to being on the face of a cube, though; that's not nearly far enough away, obviously. :)