To each their own. I've been keeping marine fish for 13 years and I've only had marine ich affect fish that are considered highly susceptible and that had visual symptoms when purchased. Many of these fish have been just fine after they've acclimated. Others have died and I've written this off as an issue compounded by the ich, not the ich itself.
As for QT, I've never practiced that either. The common ideology is that if the parasite is present, it will be caught and treated at QT. However, it is commonly accepted that fish can carry the parasite for months to years without presenting symptoms, and that being the case and the potential for hypo-resistant strains, what's the point?
There is much speculation and lack of information regarding this subject. I would say to claim anything as fact, or failsafe is a bit of a stretch, but this is simply my opinion.