Propane filling question

Brand new tanks need filled that way to insure that they are purged of air. But there is no reason that I know of to fill used tanks that way. It does waste some propane. I do not have permanently mounted tanks, but around the farm I have a flock of 20, 30, and 40 pound tanks I use for various purposes, and the only times I've had a propane seller use that method at the store I get 99% of my propane from, it was on a new tank. I, too, always take in a number of tanks at once, since there is a price break if you buy more than 18 gallons at a pop.