Why not? The most important thing about a round planet in this case (and, in fact, the reason planets tend towards roundness) is that it means gravity is (roughly) equal at every point on the surface (and, in fact, at every point on any planet-shaped shell inside it, as well). On a cube, this is entirely gone. The centers of the faces would have the strongest gravities; the corners would have the weakest. For localised walks we wouldn't notice the gravitational changes (just as we don't notice them when we walk up a staircase), but if you made the trek from the center of a face to the edge, you will definitely notice.