Will you never want to another fish? What if a fish dies, won't you want another? As long as you plan to have fish, then my opinion is YES, plan on keeping a QT. I'm the Disease Moderator here, so, to me I have strong opinions about it. If you really are not going to add any more fish, and you are not going to try and treat any fish that might get sick while in your main tank, then you don't necessarily have to have the QT ready to go. I would, however, keep what you need to set one up, ready.
I deal daily with hobbyists who have to set up an emergency QT. Believe me, there is nothing like knowing I have a QT ready to go, cycled, it can take meds, it can take fish, it can take whatever I need to throw at it, because it is cycled and a thriving QT. Folks in an emergency can set them up, but they still don't have a cycled QT, and that's a fact. They will have to worry about their sick fish being in a new tank. And a new, hurriedly set up QT can be hazzardous to fish health.