Servos generally need a continuous stream of pulses to keep them on position - so normally attach() in setup and don't detach().
If you want to power-down the servos you'll need to see if this works (they stay put on power down), and then arrange for the power to be switched somehow (as well as detaching immediately before). But I'm guessing - someone will know more, but they might want to know which servos you're using.