just kidding. you can hold it any way you want, though if you want to shoot fast when the opportunity arises, i would think that holding it by the body is preferable.
that said, some lens/camera combination, probably better to hold by lens, for example, imagine if you have a 300-800 mm, the camera looks like a toy stuck to the lens, it would probably be better to use the lens as a point of holding rather than the other way around, because of moments.
Well I hold the camera by the body only.
Yet again... I only have a 50mm lens.
So it feels pretty weird for me to put one hand below the lens.
FYI.. i hold my camera with one hand on grip, the other in a 'U' shape arnd the lens..
EOS 30D+50mm MKII+18-55mm IS.. What I use do not define me.. Its the photos that I take that will..
That's why never ever let your friends touch your camera if they do not have the knowledge of the equipments.
To them, a Nikon D3 and Canon 1D is just another camera.