I'm on the other side of the fence from Derrel.
When my camera with lens is out of my hands, it will either be on its back for changing lenses, or, on its bottom for a longer period. To my way of thinking, spreading out the weight between a larger surface-contact area puts less stress on the components. Although having some 200+ mm lens mounted and on a table might produce some added stress on the mount, my fear of having the camera lens down or even lens up makes it very top-heavy and more prone to fall. I shudder at the thought of a $6000 camera 'atop' a $16,000 lens toppling over because someone accidently bumped the table...even if it IS insured. Of course, I own neither of those, but even my gripped 5D3 with 80-200 magic drain pipe mounted will never be face-down.
I feel the same way about carrying my camera in a bag. My thinking is the same. If, heaven forbid, the bag gets dropped 24" to the floor, I want as much cushioning as possible and as much area as possible on each piece of equipment to absorb the shock. Although, for this purpose, I would like to have all my lenses horizontal when being carried, practicality necessitates that the short ones ride bottom-down and the long one horizontally. The camera is in 'normal' orientation sitting on its mounted grip. Even if I someday decide to get a small shoulder bag to carry the camera with lens mounted, it will be 'horizontal' rather than 'nose down'.