There’s nothing wrong with cropping to change aspect ratio per se but many photographers only think about the shape of their images when they’re looking at them on a computer screen. If they considered aspect ratio at the shooting stage they’d produce better composed images.
The usual guides like the ‘rule of thirds’ apply to square, 16:9 and 5:4 images in just the same way as they do with 3:2 and 4:3 images, but the compositional hotspots shift relative to the shape of the frame. Consequently, it’s better to think about the aspect ratio that would suit a scene before you shoot it. You’re able to make sure that all the elements of the image are correctly position relative to one another.
Most cameras have aspect ratio settings in their menu with options such as 1:1, 16:9, 4:3, 2:3, and 5:4. If you select a non-native aspect ratio, i.e. one that differs from the sensor’s aspect ratio, and shoot in raw and jpeg mode simultaneously you’ll have a raw file that’s cropped to the aspect ratio you’ve selected as well as a raw file with the full sensor readout. So if you make a mistake or don’t have time to change the aspect ratio setting you can rely on the raw file.
Selecting an aspect ratio in the camera’s menu allows you to see images in that shape. If you have a compact system camera the image on the screen and in the electronic viewfinder will be in the selected aspect ratio. With a DSLR you may see some cropping lines in the viewfinder, but usually you’ll have to shoot in live view mode to see the aspect ratio applied before you take a shot. If you don’t want to use live view mode you can always shoot using the viewfinder, then check the image and retake it if you haven’t got the composition quite right.
Pick an aspect ratio
I recommend picking an aspect ratio and shooting with it for a while. If you shoot lots of images every day then use that setting for a day. If you don’t shoot that much then maybe use it for a week or a month, however, long it takes to give you a real feel for what works with that aspect ratio.
Once you’ve worked your way through all your camera’s aspect ratio settings you won’t need to change the setting anymore when you come across a scene, you’ll just look at it and decide what aspect ratio will work and compose accordingly. Then you can crop the image on your computer without making any compromises.