Cropping has to be one of the most debated aspects of photography. On one side of the spectrum we have people who crop photos, and see no harm in doing so. On the other side we have people who think cropping an image is something you just don't do. I'm one of the believers that as long as you're not relying on cropping, there is no real harm in it.
There are a few reasons why you may want to crop a photo. Maybe you took a fantastic photo, but you accidentally got your horizon on a slight angle and didn't notice until you were in post. Maybe the composition is slightly off, but the photo is really good. Perhaps a quick crop would take it from a nice photo to absolutely stunning. Another good reason is you love the composition, but there is something distracting in the shot. (Although it can be argued that you could / should have zoomed in or compose the shot slightly differently not to show the distraction. I have to agree with that argument.)
Like everything else in photography, you need to strive to get it right in the camera. If you start to rely on cropping to get your photos right, you're never going to get better at being a photographer. Cropping shouldn't be a crutch - it should be a tool in the tool box that only gets used when needed. Unless, of course, you plan on taking a photo and intentionally doing a stylistic creative crop. In that case, crop away.
I've provided an example of a photo that I recently took. I absolutely loved the shot, but decided to crop. I didn't like the original composition, and I feel the photo benefited from being cropped in slightly. I like delivering high quality product to my clients, and if cropping the image every once in a while is what it takes, so be it.
My opinion stands that in a perfect world you shouldn't have to crop your photos, but realistically it is not the end of the world if you do. Be cautious of over cropping, because if your clients want a massive print they might not be happy that they image quality is poor because you relied on cropping. Always strive to capture the photo properly, and only crop of you absolutely need to.