There are photographers who prefer to standardize the aspect ratio of their images. Many documentary photographers crop none of their images to indicate the objective stance with which they attempt to approach their subjects, signaling a host of other related practices and above all that they have intervened in and interpreted the events their images portrayed as little as possible. Other photographers standardize the aspect ratio of their images to draw attention away from the proportion of the frame to other aspects of their images. Repetition typically deflects attention from an image element, while variation draws attention to itself. Still other photographers standardize the proportion of their images to make matting and framing easier and more economical. Some do it simply out of habit. No matter what type of photographer you are and what your standard practice is, I recommend you explore your options.
With Lightroom, Adobe Camera Raw and Photoshop, you can nondestructively crop your images so that pixels are hidden rather than eliminated. This is now the default behavior of these software applications. Later, if you wish to change the crop of an image, you can reuse the Crop tool to reclaim these hidden pixels and make modifications without having to re-process the original RAW file from which they were derived.
Few photographers explore distorting their images to change aspect ratio. In large part, this is because distortion is a practice so new that not enough time has elapsed for it to become a habitual part of our tradition. In some cases, the practice of distortion is discouraged; it’s not appropriate for the creation of forensic or documentary images. In a few cases, it’s encouraged; many portrait photographers squeeze their images horizontally, making their subjects look thinner (a transformation of as little as 5% to 10% is often undetectable to the casual observer, but nonetheless highly flattering to the subject, and some think it takes off the 10 pounds the camera adds on). Whether you’re influenced by force of habit or respect for tradition, remember that both lead to learned behaviors, which can be modified. Distortion offers extraordinary expressive opportunities. You owe it to yourself and your images to explore this option. Whether you choose to do this often or infrequently, a little or a lot, is something only you and time can tell.