It is good to balance and rotate your tires at least annually. Doing so will ensure that your tires all wear evenly, and thus will last much longer.
Unbalanced tires cause a harmonic vibration and will wear your tires, wheel bearings, and steering and suspension components.
Under-inflated tires are a drain on fuel economy. This same friction is what leads your tires to wear abnormally. Just with unbalanced tires, tires with too low or too high pressures will cause abnormal wear patterns, and they will wear out much more quickly.
Aside from the annoyance of holding the wheel to one side, improper alignment will wear your tires abnormally and much more quickly.