[All right...to add fuel to the fire. I've only tried to QT once - and it died after 14 days. Not due to starvation, not due to stress. Due to an ammonia spike. I had a BEAUTIFUL powder blue tang that was eating, active, fat and healthy. My QT was an 18-gallon rubbermaid with some live rock and water right out of my 120 display tank, so I know it was cycled. I ran a Penguin filter on it big enough for a 75-gallon tank.
I was going to release him into the main tank on the day it happened. I left in the morning to run some errands...he was fine. I came back 3 hours later and he was flipping out. Tested ammonia and it was SKY HIGH. Wham, bam, dead fish.
I put a copperband straight into my main tank, flipped off the lights and the next morning, he was just a tiny bit icked up (only 5-6 spots). But otherwise active and healthy. My yellow tang chased him a little, but he adapted well. I didn't QT him in the first place due to the starvation issue, plus I had just had the bad QT experience. He icked up a couple of times over the next week, but some days it would be there, some days there would be 5-6 spots again. The butterfly was fat and happy until the now infamous tang killed him - I was watching so I know that this is what happened.
I'm getting another butterfly this week and will not QT him as I believe that with this particular kind of fish, the double move will cause more stress than harm and also because of the starvation factor.
My personal MO is that I will QT or not depending on kind of fish, what's already in my system and special needs. Right now, there is nothing in my tank to cause a new fish stress - no bullies, etc. But I know that my display tank is a lot more stable than I could make any QT system. I'll go with those odds first.]