Beth states on the BASIC FAQ sticky at the top of the forum:
Should I treat my fish even if I don’t see any Diseases?
You many also want to perform hyposalinity in the QT—regardless of the presence of parasites [ich] on fish. It can’t hurt to do it, can only benefit, but, then your QT time will be extended. You will need to reduce specific gravity/salinity over a 48-hr period, maintain the fish in hypo for 3 wks, then, over another 4-5 days bring the salinity back up to your main tank parameters. So, you are looking at another week in QT, in addition to the standard 3 wks. Hypo is a very precise process, however. You must get the specific gravity down to 1.009 and keep it there. It must not rise above that. To accomplish this, you need a refractometer or a salinity monitor to measure the salinity. Other instruments are not as precise, but you can also use a quality glass hydrometer.
Of course you don't have to do this as you suggest.