I dont qt plants. I know some people do, but I just rinse them off real good and acclimate them to the tank.
As far as a qt for new arrivals, you definitely want a cycled tank for that. Running a small filter on your main tank, or having a filter that you can pull some media out of will work good for that. No need doing the whole fishless cycle route. I know some people run a small HOB
on one of their tanks so that if the need arises, they just set up the qt, add the filter, then bring home their new fish.