I carry my DSLR in the same bag I used to carry my SLR, and there its position is screen down. I've seen smaller bags, adapted to the shape of the camera, where it stands objetive down. What I wonder is if the storage/carrying camera position matters, for dust depositing on sensor (camera care), or for other reason (is easier to fastly take the camera and shoot).
The safest position for your SLR is in your bank's safety deposit box.
Once you decide you want to take pictures, you must accept some non-zero risk of damage to your camera.
Camera in bag with lens facing up:
- Harder/slower to remove camera from bag.
- If bag hits ground, you must ship the body off to repair the screen
Camera in bag with lens facing down:
- Big handle to grab camera with... faster to shooting position
- If bag hits ground, you'll likely need to send that lens in for repair
- You might have been able to avoid damaging the body's lens mount. If so, you can mount another lens and keep going.
Every increase in camera safety comes with an increase that you will miss the next shot. Everyone must decide on the balance they're comfortable with.
On a personal note, I've been lugging SLRs around for decades without a bag and have yet to damage one beyond a few scratches/dings on the body.
Admit to yourself that you'll likely lose complete interest in this SLR the moment the next new shiny goes on sale. Choose life. Be careful and go shoot pictures.
I don't think it particularly matters. I have multiple different cases from Canon and the default configurations of the cases put the camera in different orientations in each. My shoulder bag puts the camera level with the ground with a small lens attached, just like it would be if I was about to shoot with it.
MY backpack offers two different options, either the camera body oriented so that it is aiming off to the side of the bag with no lens attached and with lenses going up and down your back or it offers a slot where the camera can be stored at the top of the bag (near your head) with the lens facing down. In both of these cases, when placed flat to open the bag, the camera body would be right side up.
It is also worth noting that both of these bags are designed to be highly reconfigurable so that you can pack things however you like and pack them securely, but the configurations I use are how they come from the factory. I only update the layouts to support the size and shape of my gear snugly.
Given that there isn't any particular consistency even in the way Canon themselves design their camera bags, I don't think it particularly matters how you store it so long as you ensure there is sufficient padding around it and that it is held securely. Getting it out quickly seems to be a bigger priority in both Canon bag designs.
Lens should be facing DOWN! Because lenses breathe, pull in dirt, particles, etc. If you store lens facing up, then all these fall into the camera body, hitting mirror, and maybe jamming mechanics. This is even more true with a cheap lens: if a small rubber/plastic/etc. gets dislodged, it will fall into the camera body...
The safest position for the camera to be in is almost definitely the most secure position.
If you've got a shoulder bag chances are good there's a "bottom" that always sits on the table, floor, or ground, so it's easy to argue that one position is "up." Other bags, like a messenger style or backpack are likely to be set down on the bottom or back -- so which position is "up" for those bags? If you've ever set your bag down in a bad location it's no doubt been knocked over, so does it matter if the camera is "up"? In all of these scenarios, the important thing is simply that the bag hold the camera securely so that it doesn't fall out and is adequately protected from all sides.