Besides making for some neat looking animations, this will also lower rendering times. For a render that would normally be 768x482 or so, make it 768x380. Leave the ratio just as it is (6:7). Any where from 380-330 or so will look pretty good.
Remember though that Imagine will still show the entire normal view of the animation in the Stage editor's preview window. This means that while you can see objects at the top of the screen here, they might actually be cropped in the final rendering.