Great question and I'm on the front end of this myself: my main DT (105g tank + 35g sump) just completed its cycle, and my fish QT (20g long) is almost cycled. I'm going to introduce two clowns into the QT soon--prophylactically treat with hyposalinity, Cupramine and PraziPro--and then gradually introduce other fish after the clowns are released into the main tank. Why would you need to break everything down after each fish leaves the QT if you are treating to ensure that the fish is rid of relevant parasites and that you are about to release those very fish into the main tank? In other words, wouldn't the bad critters, etc. (hopefully none) be on the clowns that I just turned loose into the main tank? Not being defensive, I just don't know and here to learn.
I was considering a large water change to get the QT ready for the next fish. I can't stand the thought of another cycle for the QT . . . ugh.