As you probably already know, it depends. Aerodynamics doesn't matter much at the speeds we typically drive, so I suppose the more limiting factor would be the uses you intend for the curved spaces on the interior. Curved upper cabinets are obviously less commodious than rectangular-section cabinets. Do you plan to put lighting in the ceiling? If so, where is the best location and does the curved section make a difference?
These are only a few considerations. In the end, it's just what you like and what you wish to do with the volume that you have to work with.
For me, it's more helpful to work on the interior plan first and let that influence the exterior configuration. Find out what stuff has to be where and what clearances you need, then you will know how big a wrapper to put around your stuff.
