When it comes to the middle vs. side portion controversy, a round face shape gives you the best of both worlds. A symmetrical and extended effect around your face can be achieved with a middle part. Curtain bangs (parted in the middle and angled down, increasing longer towards your jawline) retain the focus on the middle of your face, providing a longer, more diamond shape effect. More information on how to style curtain bangs can be found here. Choose a deep side part if you have a round face and want to know how to part your hair without going down the middle. The same illusion is created by a deep side part as it is by a center part. Hair may still be done in any way you choose with the side part, whether it's a power pony or classic, beautiful waves.
With a longer face, a middle part cut with bangs looks great. Layers or fringe can also help to shorten and frame the face. The bangs (in any style) will allow you to part your hair as you like. If bangs aren't your thing, a middle part with layers or a shaggy or fringe to assist round and soften angles can suffice. If you like a less layered style, go for a softer side-swept or diagonal part with a bit of volume at the part. For long faces, this hair part produces a rounder appearance by shortening the forehead and chin characteristics.