Centering the subject
Centering your subject is taboo in some photographer circles. It is seen by many as an amateur mistake. However, while you should never just automatically center you subject without thinking, there are times and situations where centering your subject makes perfect sense…
Centering is an effective way to draw attention to your subject when there are very few other items in the composition. When taking photographs of stand alone objects where background and foreground are not shown and the subject will nearly fill the frame, centering works to maintain focus on the subject itself. Portraits (animal and people) and illustrations are good examples of this. Also, centering can be used to good effect on the other end of the compositional spectrum when the composition is extremely busy. When there a lot of objects in a frame that compete for attention, centering a strong and different type object can draw attention to it. This works much like when you are working a jigsaw puzzle and are faced with many similar pieces. If you place a puzzle piece of different size, color, or shape in the middle of the group your eye will be drawn to that difference.
~Liz Masoner from her article Centering Your Photographic Subjects