I recently spent a few hours with
Hucho, and I strongly recommend it as an introduction to vehicle aerodynamics. It contains many illustrated examples of different configurations tested in a wind tunnel.
You can gain a small improvement in drag with a rounded front end, and a large improvement in drag with a tapered rear. Low-drag concept cars invariably include both.
But I think this is a bit academic. You're not going to take the cubicness out of your Cube, are you?