I may be wrong (pretty sure i am not) but I have heard numerous times that if you lower the salinity or raise the salinity to do any good in regards to parasites (ich, velvet, etc) you would have to raise/lower it beyond what the fish can handle to do any good. This would mean that hypo is pointless, fixes nothing, and causes undue stress to new fish. If you are doing it for some other purpose please elaborate.
I did once have a problem when a pair of tangs came in with velvet... I was told to give them a FW dip daily which causes the cysts to fall off the fish, and then put them back in the tank. I also ran a UV which helps (some say it may not) with killing the free floating stage of the parasite. Either way you need to treat for up to two months to insure that all the parasites are dead and gone. I did also run copper at the same time. One of the yellow tangs survived but velvet is almost impossible to win against unless you catch it early enough.