Flush it. My house is from the late 70s and I don't worry about it.
As long as you clear the lines with fresh water you have nothing to worry about. It's not like it will sit there and rot the lines as long as you give it a little rinse.
I run my old tank water into the toilet through a hose.
I run a few gallons of fresh water to flush my water change hose. Then I flush the toilet once after to reset the level in the bowl.
'nuff work for me
