I go to the exchange places to get a new #20 tank that's good for another 11 or 12 years... For $20, I get a new tank and some propane in it... When I empty it, I take that tank and refill it like you do for your big #30 tanks... ;)
Ever price a new #20 tank from the store.. $30 at least, then you have to now fill it!
All the exchange places I go to, I can pick the tank I want, and I just look for one that has a recent date stamp on it.. Pretty simple. ;)
But, do whatever works best for you.. I've got 3 other #20 tanks at home that I use for the bbq and Mr. Heater and a spare. Plus, if I need one, I have pulled a tank off the trailer when I needed it for the house.. Then I get it filled when I can.
I've gone to the exchange place 4 times now. Still have one more that's within a year of needing to be replaced. Guess where I'll go at that time? :)
Mitch