How old is your tank? If it's well established and your fish are healthy I wouldn't recomend QT for all new purchases. In this case however if you see sick fish in the system, take a pass. A QT tank can be a big stress, usually they are smaller tanks and water params can change quickly which can have grave consequences.
I don't QT and in 30+ years have never had an ich outbreak, that's not to say I haven't lost a few fish to Ich and the stress it causes, but I keep my fish healthy and my tanks equally so. Haven't had a new fish pass anything on to the other fish in my system. That's just my experiences and my opinion, not saying that it's the way to go, there are many paths to take. Some people lose healthy fish from the stress of a QT set up. Good luck either way.
What kind of fish were you looking at? Some are ich magnets like achilles and powder bues tags.