Excess heat can shorten the life of your computer. The most common causes of excess heat are dust and dirt. Clogged vents can keep heat-dissipating air from moving through the case.
If your computer is in a clean environment, an annual cleaning should be adequate. If your computer is in a less that perfect environment, you should clean your computer more often. Always turn off and unplug any components. Liquid cleaners should be applied to a soft cloth. Never apply a liquid cleaner directly.