While I do agree w/ Beth, I went a long time in the hobby before I Q'd fish purchases. Unfortunately I must say that my death rate on the actual purchased fish between using Q vs not using Q is pretty much the same. The big plus is that if you do intro. sick fish directly into display tank, you have to treat the whole tank. This could be very tough if you have inverts, L/S, L/R etc. If you don't I'm, not sure what I prefer, Q'ing or F/W dipping with direct intro. The latter has been as successful for me as the former. Just my experiences.
By the way I just lost a flame angel that Q'd for over 3 weeks. SOmetimes good intetntions still end up bad. I believe inthis case it didn't matter whether it was Q'd or not. My theory is that it was caught by usinf cyanide.