The rule of thirds is a well known fundamental photographic principle of good composition, and it's one of the first things that budding photographers learn in classes, and rightly so. However, it's only a guide, and ignoring the rule doesn't mean your photos will be dull or unbalanced.
Apparently studies have shown that when we view an image, our eyes do not immediately look at the centre of that image. instead we first look around the sides of the image. This means using the rule of thirds works with this more natural way of viewing an image. If you were to draw two imaginary lines at third intervals horizontally across an image, and then 2 more vertically, where they intersect is quite often the ideal position for placing your subject or an important part of the composition. Compare the above photo with the one below, and you can immediately see that the first one looks better.
Ok, I admit, that was cheating a bit because I chopped off the lovely looking sky there.