it depends on the type of QT you run and the type of fish you're QT-ing. we normally have QT's up and running 24/7 and these tanx are basically fully set up and cycled w/LR, substrate, and (usually) macro algae. if we know we'll be treating a fish prophylactically, the fish goes into a BB uncycled HT. if we go the HT route, we hang an ammonia alert tag in the tank in addition to daily water testing "just in case".
to minimize stress, you can use fake coral, rock, or even PVC to give the fish some comfort if you run a BB setup.