i personally will QT any fish that are not wrasse, mandarin or gobie.
man not trying to flame but there a lot of shocking responses on here, and it seems to me a lot of you guys are skating on thin ice. i have seen 2 ick outbreaks from putting in fish with No QT, and the other from putting in a fish that was 3 weeks into a QT period. as mentioned earlier, if you really believe that a LFS (unless you worked there) will hold your fish in a true QUARANTINE tank - not in a show tank with sand in it - for 6 weeks, then i may have a bridge or two to sell ya. Even if they kept the fish in a tank for a month, a) usually that tank will be connected to a portion, if not ALL the tanks in the store, hence all fish are exposed. and B) the fish could have it in small amounts that wont come to fruition until stressed again by being moved to your tank. and C) if the fish didnt have it, the stage of the ICK life cycle that thrives in calcerous material before it leaves to find a fish to latch onto, will be alive and well in any sand your fish is swimming over.
even from purchasing fish online, from here and elsewhere i have seen it. I have seen ich appear 12 hours before the 4 week period was up. if i didnt QT, a disaster would have struck. i have gotten fish that have had black ick, and lymphocystis - so what then. all of these symptoms appeared at least 1 week after i got the fish. If you dont care about the amount of money you have invested in fish live stock and the well being of your pets, than by all means, drop em right in there....