I have a small saltwater tank running empty, for adding anything to my reeftank. Freshwater I don't have one at the moment, because I haven't bought any freshwater fish in over 6 months. When I have a setup that requires it (larger community tank where I am still adding fish, etc) I will run a freshwater QT tank and usually QT fish for about a month. If I have a setup where all fish will be added from same source at same time, Like for example if I'm growing out a group of same species cichlids, I don't QT, since anything they may have, they have all been exposed to already, and won't be risking the health of anything else.
For anyone running a tank where the stock will change around over time, I definitely recommend a QT tank, can save a lot of headaches.