If I buy a fish from a store it gets quarantined, if it comes from a friends tank and I know it's healthy I just acclimate with lights out.
Recently bought a filefish right out the box I didn't have a month to quarantine and so far so good. I too have never had disease in any of my tanks my theory is if your tank isn't overpopulated and your tank is healthy them your new fish will too. I'd never buy a unhealthy looking fish though I should also add
