I have good intentions to QT and always tell people to QT, but in reality, I almost never do. I am terrible at that. Mainly because the only spot for my QT tank is in the spare bedroom, which is mainly used for storing junk right now. It's really hard to get into (the tank goes on the built-in dresser on the far side of the room, so I hardly ever go in there. It kinda defeats the purpose of observing them, etc. Plus, that room is very cold in winter and very hot in summer (trailer house, ugh), so the tank temp is hard to regulate. Once I get my mom's tank to her, I will probably move the QT tank out there when I need to use it. That is a much better spot in the dining room, where I would walk past it several times a day but still be in a relatively quiet area.
So, yeah, I'm a bad girl. Fortunately, I have not had any major problems, but I do believe that it is good practice to QT every fish. I mainly get my fish from trusted small LFSs, and they usually have good stock. But, it's still an unknown quantity, really. There's still a chance that there is something lurking. If I happen to buy from Petco, Petsmart, etc., I always QT them no matter how nice they look. I just don't trust their suppliers, and most of the time the staff don't know what they are talking about.