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This website suggests you taking photos with your digital camera upside down to get the best results. It has to do with lighting angles and facial features having a better look when the camera is upside down.
Read on:
http://www.digitalcamerau.com/2006/0...ter-portraits/
Site contains a sample of this but to be honest I could barely notice any difference at all... but perhaps if you try it yourself you'll notice the difference.
From a "professional" point of view, this will make a difference, with some of the cheaper lens's that are out there. Many are set in the mount without any regard to the correct correlation of light bending. Think of astigmatism in the eye. The same thing. The better quality lens's such as those from Leica, Nikon, Cannon, and Schneider-Kruznach, to name the top 4 (in order) take that into consideration when the lens is assembled. With any of these you will not notice any difference no matter how you hold the camera.
I find it easier to have the subjects stand on their heads. :D
from a photographers point of view I think having the flash under the lens will "fill flash" the face better so you dont get the raccoon eyes and rolls under the chin and neck. I use a flash bracket to get the flash off the camera and have a soft box on the flash