To answer the original question, I have QT'd some tough fish over the years including fish that you cannot put through copper, such as puffers. I do not prophylactically treat fish.
I keep a 40G breeder QT always running with HOB filter, live rock, ect. All fish start in this tank for 4 week observation, minimum. If I don't run into any issues after 4-6 weeks, then into the display. If I find internal parasites (most common thing I run into) then I treat with metroplex/prazipro in this tank. If I find ich and I cannot use copper (puffers) then I go hypo in this tank.
Anything more serious and I set up my hospital tank, which is another 40G breeder, but is not always running. Here I can use more serious meds.
If I find any parasites in the QT, I keep it fallow for 45 days at 81 degrees. I know in this time I cannot buy more fish, which is fine. I buy in groups and never impulse buy.
I used to HATE QT and fish would often die in my QT anyway. I was doing it all wrong, usually setting up a QT the day before I bought fish. I was dumping them into a stressful situation. Now I always have my QT running and cycled, which has made all the difference.
I also combine hypo with TTM sometimes if ich is really bad.
Anyone saying you don't need to worry about ich is wrong. I agree, it won't often kill fish, but it can and there is no reason to make your fish 'just live with it'.