Yes, unless it comes from a known personal tank, or it's a certain species that is know to be resistant, and shows no signs after a few days. For the first time in years I did take a chance on a coperband that was eating, but not to well. Who showed signs of ich. After 2 weeks of hypo and frequent water changes in that short qt, i put him in display because it was getting skinny. That was 2 months ago, and now it's gaining weight and so far no signs. Got my fingers crossed.
It's hard to know or sure if qt is necessary. With so many ppl with years of experience with no issues and beautiful tanks. Who say diet and and a happy stress free tank makes for a stronger resistance and no issues. Healthy tank, healthy fish seems to work for many. I have been down the ich nearly wipe out the tank road, so learned the hard way to qt. But looking back have to wonder if the tanks newness or minor issues didn't cause it to bloom.