I QT all fish, whether I do it myself or I purchase them from Adam at Ocean Devotion. Corals get throughly dipped and frag plugs go bye, bye. I would QT or hold/observe all CUC, especially from Reef Cleaners (which is where I got pyramid snails from). I never use to QT CUC, just acclimate and go, but after battling pyramids for a year, that won't happen again. Clams will go into a QT/observation tank first and any signs of pests will be eliminated there and I can treat for PMD if needed before going in with other clams. Wild caught clams can wreak havoc on established cultured clams, so know your source. You're just rollin' the dice if you don't QT or observe at minimum. I treat every animal as if they came from an infested tank/system, it's much easier going forward with that mentality, then it is going blindly and trusting someone's word.
But hey, you can do whatever you feel comfortable with, so there's no right or wrong here, just personal preference and experience will guide you.