It's great that several LFS have established QT procedures for new stock. However, their QT regimens are not a guarantee that the fish you purchase carries no pathogens into your display tank. QT is more than simple prophylaxis and/or active treatment. It also provides your newly acquired critter time to adjust to your feeding habits as well as time for you to observe the habits of your new fish. As was mentioned earlier, unless these are captive bred fish delivered directly from the breeder (think ORA or SA) to the retailer, they have been through the hands of collectors, wholesalers, the retailer and then you. Your new fish needs some rest and relaxation before you drop it into a tank with potential rivals for space and food.
Failing to maintain a QT tank is simply inviting disaster. A simple bare bottom 10g or 20g tank, PVC hiding space, sponge filter and a heater are all that is required. Your system doesn't have to stay running 24x7, simply drop the sponge filter into your sump and transfer it to the QT tank when needed. Viola! Instant QT. Once your QT regimen is finished and you've transferred the fish to your display tank, clean the sponge filter, let it air dry and then return it to your sump for re-colonization by nitrifying bacteria for your next fish purchase.
There is no such thing as a disease free fish.... fish become susceptible to disease when they are stressed... much like you and your immune system. Similar to getting a flu shot for yourself, QT'ing your fish doesn't guarantee that the animal won't get sick, but if it does come down with something, treatment and feeding will be much easier in a small system... and you don't risk the rest of your prized collection by playing Russian roulette with them.
I have a lovely fish in QT now... it's been there for 6 months.... it's eventual home will be with a pair of Tinkers butterflies and Interruptus angels.... what do you want to bet that when I move it... I still might see a little white spot? However... it will be well conditioned and be able to handle the transition.