Think of your aquarium health the same way you think of the health of a severely immunocompromised child. For these children, a simple cold could be fatal. What procedures would you want to occur before letting a guest into your home? Would you trust the guest if they said they followed the procedures but you could not verify? That is how you have to view these things. In this hobby, people have to learn the lesson the hard way, typically a very expensive system crash or mass causality event before they take these thing seriously. If possible you want to know the specifics of the quarantine process followed by your suppliers. You want to know the details of the medications or dips they do. Then make sure all of that aligns with a risk tolerance you have.
It is amazing how patience and discipline can avoid many of the "difficulties" in this hobby. Take from those of us who have learned the hard way.