The rule of third of photo composite means that we should place the main subject in the left or right third of an image, leaving other two thirds open to add something such as writing, comments, etc.
The goals of such a photo composite is not only for flexibility, but also for the viewers to capture the main subject in a photo immediately, because the human eyes were placed on approximately one third position of each side of face. It would be easier to notice anything straightforward.
I have been following the rule of third most of times, to make sure leave some empty space on an image.
However it can be also striking if we place the main subjects in the middle of photo frames to emphasize the story.
The rules were made to follow or to break, which is totally dependent on the stories a photographer tries to tell.
From Hiding to Blogging 