I agree with Dan, the effort needed to accomplish what you are asking might not be worth the savings you would yield. Don’t forget the power supply will inherently draw some power just being plugged in. Here is some data (http://www.raspberrypi.org/phpBB3/viewtopic.php?f=63&t=6050&p=291334&hilit=watts+power#p291334) on the power draw of the Raspberry Pi with and without a wifi dongle. The power draw at idle was half a watt which is what a power supply will draw with the Pi being off. I really don’t think you would see ANY real world savings doing what you are discussing.
With all of that said, you could argue having it turned off most of the day might reduce heat damage if the controller is outside. However, the Pi can handle a fairly high temperature from my understanding without failing.
More data: http://www.kaibader.de/my-new-raspberry-pi/