Exactly four years ago, 15 July 2011, I wrote this – and it is still relevant. Plus ça change…
When you are using an SLR to look at images you have taken on the back of your camera, set your camera to not autorotate the images. That way you can see the image fill the entire LCD instead of part of the LCD with big black bars on both sides. And that looks so much better!
On some cameras (all Canon SLR cameras, for instance) you even have two options: “turn or not turn” on the camera display, and “turn or not turn” in the image itself.
In this case I set autorotate ON in the file, but OFF when reviewing on camera (i.e. I use the middle option: the file is unchanged, just the displaying on the camera changes).
You will find thise fuction either in the playback menu of your camera, or in the settings menu.