Prophylaxis is one of the controversial topics here. I had "observed" my first set of fish, 11 in a 110 gallon. 3 months after the last fish went in, so that's 3 months of "observation" with the fish looking good, they started coming down with Ich. So now I treat all fish with hyposalinity regardless of appearance.
IMO, it is a QT that is improperly set up that stresses and kills fish, not the prophylactic treatment. If you can get your fish through 6 weeks of QT, then you can get them through hyposalinty just as easily.