I agree, I used to quarantine and treat with copper, but I now just simply quarantine, and let them be. What I have also done that I think is a big plus, is finish the fish off in my 65 gallon fuge for a week prior to introducing into the display. My fuge has a huge ball of cheato, which the tangs really go nuts for. I can also over feed a bit, since the fuge is part of my system and has plenty of filtration. This gives the fish an opportunity to power feed and put on a bit of weight.
Even if they hide for a while when introduced to the display, they can afford to miss a few days of eating.