Depends on what you mean by “efficient use of space”, I guess.

If you are organically building onto the ship or continually lumping new tech together, you’re not going to end up with straight, clean edgesor uniform proportions. And assuming intra-ship distance matters for things like power/resource distribution, the natural shape will be spherical.

Now if the borg need to pack dozens of ships into a spaceport or something, a cube starts to make more sense…