I would never do the exchange thing. It is an expensive route but it is convenient at times. I always find the discussions about getting ripped off interesting. Some folks are quick to point these kinds of things out. I choose not to get all worked up about this because, if you pay by the gallon you will always pay for what you get, no more or no less. When you trade in you might have1 or 2 gallons left. You trade in for a full 5 gallon tank only to find out that it only has 4.5 gallons. So, you paid full price and only got maybe 3 gallons.
Avoid trade ins and you won't have to worry.