Maintaining a balanced lifestyle in this fast society, is the key to survival. But already in the ancient Yin and Yang theory, balance has always been the basis. However, if you use the rule of thirds in photography, you place your subject off-center, out of balance. This leaves a void space around the subject (called negative space), and that could result in a empty canvas with only one subject, and turn out rather boring. In that case try to fill this negative space using formal or informal balance.
Formal balance, is when the space directly on the opposite of your subject is filled with an (almost) identical shape. Or, if you could split the picture in half, and get two similar halves. This is also know as a symmetrical balance. Both subjects are identical (like a reflection).
Formal balance, can also be achieved by centering your subject. Corresponding in the picture below.
Another example is the picture below. The red surface of the empty space on the right and left side of the bottle are equally big. Creating a balanced composition.
If, on the other hand, you balance your photo with two unlikely shaped objects, this is called informal balance. Take a look at the next photo. The poppies in the bottom left corner, are off-center. But the bunker, in the right top corner, restores the balance in this scene.
Both types of balancing are used to achieve the same goals, to create a tranquil balanced composition, that is pleasing to the eye. So, is it a rule to balance? Like always the answer is no! You decide what is best for your photo!