I think you mean a voltage meter rather than a voltage regulator. They are useful things to have around. If you turn off all your power and see that the electrical meter is not running, you won't need to test voltage to the switch. Otherwise you need to be really sure that you turn off the right breaker. The labels on the breakers in the electrical panel may not be complete or correct, so don't rely on them to tell you which breaker serves the faulty light switch.