It depends upon the design of the bag.
I have seen bags that have a long round section to accommodate the lens, some of these have the lens pointing down.
If it is just an open area for the camera, I place the camera on its back at the bottom of the bag (lens pointing upward).
No scientific reason.
It just gives me a good place to store the lens shade (if so equipped).
I remove the lens shade, reverse it, and place it back on the lens.
i have recently bought canon eos 600d .. but i dont really know how to keep the camera in bag .. upside down or what ??

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