I never used to but recently with my obtaining some infected frags and seeing certain things come off of these frags I have decided to QT everything.
I fresh dip, QT, and then I inspect and inspect and then inspect again.
i don't keep my QT tank set up. i take a small 2.5 gallon plastic critter cage i found with pH, temp balanced, newly made water mixed with some tank water with circulation and an airstone and let them sit for a while. no set time really, just keep looking till i see anything that may need attention and go from there.