I found that it's better to shoot in whatever format you intend to print in. Otherwise, when you crop, the new format may not allow you what you want. You'll start cutting off feet for example. I also found that whatever format you shoot in, your eye will adjust very quickly to fit the scene to the camera's format.
I discovered this accidentally when I started to make vacation slide shows for my TV and monitor which have 16:9 formats. The video clips were 16:9 but the stills were 4:3 causing black bars on the screen with the stills. So I tried to crop all the 4:3s to 16:9 and had loads of problems. So one day on vacation, I switched the camera to shoot stills at 16:9 just like the videos. It took about an hour to adjust my compositions, maybe less, and then all my shots and videos were 16:9.
So what I found is I could switch from 3:2 to 16:9 to 1:1 or whatever, and I'll format the subject to the cameras' selection. It's really no more difficult composing in any format you select.
Here're a sample of pictures in the Southwest done on in 16:9. At first look, it doesn't seem apparent it's 16:9. The composition fits the format.
Explore this photo album by Alan Klein on Flickr!
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