dcheckoway, Yes, in most cases this will work. The 'hold' voltage is mostly a lot less than the normal voltage. But it depends on the relay. The example uses 50% PWM value, that seems a good value to me.
If you have a variable power supply, you can find the 'hold' voltage easily. Or you can use the Arduino with different PWM values.
But I would advice not to use the lowest value, it could depend on the temperature, on the voltage of the power supply and so on.
Suppose a 12V relay needs a hold of at least 25% PWM, I would choose 50% PWM to be safe.
The PWM frequency is almost never a problem.
fungus, Yes, so the Power supply must be decoupled very well and then it won't be any problem.