Fish depend. All Angelfish and Butterflyfish (and Tangs, if I got another which is unlikely) get a PraziPro dip before entering the QT, they almost always arrive with flukes. After they start eating, they go through a treatment of copper (Cupramine). Some fish where it's hard to notice disease, like lionfish, get a hypo treatment after they start eating. My hardier fish go into copper immediately. Something fragile like a Leopard Wrasse, goes into a specialized QT, and I just monitor and treat only if necessary.
Corals, admittedly have sometimes have been a calculated risk. Buying from someone's personal tank, but I notice how their fish look and if they seem healthy, they were straight added. Now I have a QT that supports them, they are always QT'ed first. Just fishless for 6 weeks, no treatments.
Inverts... I really don't do inverts... They could go into the coral QT...