My standard practice with most of my photography and especially car photography, is I seldom crop images after capturing them, preferring to do so in-camera in order to extract the maximum image quality from the maximum number of pixels. This is especially when shooting with the smaller sensors found in Micro Four-thirds cameras— but I make an exception to this rule for some of my drag racing photographs.
At the track, the one you’ll notice right away is that there are lots of people—spectators, crew, and safety staff—all of whom at one time or another will inevitably walk into your frame. That’s when I crop, ever so slightly while maintain the image’s original aspect ratio. Aspect ratio is the relationship between the height and width of the image and is usually expressed by two numbers. Traditional 35mm film cameras and full frame digital SLRs, for example, use a 24x36mm (3:2) format but these day many cameras shoot in different aspect ratios, including the widescreen 16:9 HD video ratio.
The way that I typically crop images is to set Photoshop’s Crop tool using the proportions of the photograph using numbers shown in the Image Size (Image > Image Size) command. In Photoshop CS6 and CC you have the option (menu at left) to maintain the “original ratio” or pick a bunch of others, including the 16:9 that’s one of my favorite options when I do crop. Using the original ratio maintains the same look and doesn’t look cropped.
Some of my photographer/friends like to kid me about this seemingly purist approach (when I am nowhere near a purist in the rest my work.) The most important part of the last sentence is this is the way that I do it. Since this is not a “my way or the highway” blog, you should crop your photographs any way you like.