When did we arrive at a point that only certain parts of a scene have to be included in a picture and everyone has to learn what those parts are and when to include them and when not to? Who gets to determine precisely what has to be in a picture and what doesn't have to be and where can I find that list? I know that you aren't supposed to cut people's ears or part of their heads off and that if you're shooting a landscape you want to leave enough room for some cut lines but beyond that, where are the rules written? I can't count the numbers of books I've read on composition and it kinda seems to be up to the shooter after a certain few things are covered, is it not???

A way of looking at 'Fill the Frame' that no one has talked about in four pages of comments is something I teach all the time. Filling the frame for me means making sure I only include what is important and helps tell the story or completes the composition and at the same time making sure nothing extra sneaks in. As to specific cropping, how do you know all the uses you might find for your shot in the future? Example: 11x14 prints sell at the Gallery. People know that's a standard size and they can get a frame for it. So I make those. But for myself, I like 11x17, so that's how I crop the same picture for me. If it's going into a show to be shown on a 72" or other large monitor you are dealing with a 16x9 format-wide but skinny top to bottom. I like to be able to fill that TV frame. I don't like big black edges, so I try to leave enough room for that. I sell note cards, both horizontal and vertical. Those are strictly 5x7. Yet another crop. One photo file, multiple crops and uses.
Some here have said to get it right in the camera, but right can change depending on use. Also, I take about 11,000 pictures of whales doing various things, every summer. Much of what they do happens unexpectedly, fast and often behind you. If I can just get that Breech sharp, reasonably exposed and in any part of my frame, I can crop it later to make the picture I wish I had.
...Cam

Once again I see misinformation about 'filling the frame'.

For some 'filling the frame' means user the sensor to its fullest in order to use every pixel.

It does not work that way. Regardless of what your end purpose it every capture needs some tweaking (sorry SOCC guys and gals). Tweaking maybe minor (rotating slightly) if you just post on the WEB. This rotation creates a crop so you lose some of your 'precious' pixels.

More often than not you need to crop to a set size that will force cropping. If you have 'filled the sensor' that will to cropping part of your image ans basically destroy it.

Whenever you shoot anything think of the end result you want, not filling your damned sensor. This means your composition must respect the end result and take into account the cropping (and rotating) you will need.

We had a case recently where printing involved such cropping and destroyed the capture. The op has likely learned something.

PP solution.... Post processing is not about correcting such mistake (or any other for that matter) PP is made to enhance a capture, nothing else unless you enter 'interpretative photography processing' that includes compositing and the like.