There's a general rule that when you have a subject that could be said to be facing left or right, you should leave some space in front of it so it has "someplace to go". This goes for any person, animal, car, flower, etc that you shoot in "profile" (or from the side).
The effects of this should be fairly obvious in the photos below:
The photo above makes most viewers a bit uncomfortable although they may not be conscious of why. Most of the time this is not a good way to crop or frame a subject that you've shot in profile. It's much more desirable to leave some empty space in the direction that the subject is facing so they don't seem "trapped" in the frame. The photo below seems more "comfortable" to most viewers.
Like most "rules" in photography, this one can be broken effectively from time-to-time. When framing a photo in a way that breaks this rule, just make sure you know why you are doing it. Sometimes backing up or zooming out a bit so you can get space on both sides of the subject (especially if it's moving) is a good idea so you can play with the crop later in software.
If you don't understand something or have trouble with any of my tips, feel free to contact me via a comment on this (or any) article, or see my website for my email address and I'll be happy to help you out!