I qt corals that I am unsure of the source (if I dont know the quality of the tank they come from) there are tons of nasties that can ride in, especially on zoas like spiders and nudis. even the rock they are on can host aip, doesnt take long for aip to open.
It depends on where your gettin the corals, and what kinds of corals. If they are from a local reefer I wouldnt bother QTing them, but if they are from a LFS you should consider QTing them, and deffinatly QT them if they are SPS from a LFS.
Feduke