Any of the water, whether waste or pure, should be good to drink because it is the water coming out of your tap anyway. Even the waste water would be better than what is coming out of your tap!
I'm not sure I understand the question about turning the valve on each time. You will want to make the water at least a day in advance in order to mix the salt and get it to the correct temp. Once you do the water change you can start making some fresh RO
, so that is when you would turn it back on (just don't mix it with salt and let it sit for a week).