Most ppl don't bother with a stop valve on the loop/gpu side and only stop valve the drain port. - reason why is if you accidentally close the stop valve on your loop/gpu side you'll stop any coolant from running through your system!
Also, when draining the system, you don't need to turn your PC on or run the pump, as the liquid will all fall out of the pump/res anyway even with it off... all you have to do is tilt the case back and forth until all the liquid falls out! ;D
Oh and one more tip, open a port/hole at the top of your reservior if you can when draining your system, it will allow air to get into your loop and help the coolant come out the bottom. - just remember to close that hole back up before you turn the system back on!