Electricity can kill you, and anything in your home that can pose a serious risk to your safety should be treated with the respect it deserves. Even though you think you've got that ceiling fan installation under complete control, check to make sure the wires aren't live before you proceed. The label on your electrical box may be mismarked or the wiring may be more circuitous than you ever imagined, especially if you're dealing with an older home. Heck, your wife may not realize you're in the middle of a honey-do project and turn the electricity back on by accident.
Whatever the reason, use a voltage meter whenever you perform an electrical repair because it's better to be safe than very sorry -- or worse -- later. Aside from being a great safety device, voltage meters are effective diagnostic tools that can help you analyze everything from an ailing light switch to a sadly silent alarm clock.