The trouble with a shuttered camera like the QHY9 though is you have to shut a min 3sec flat, then it takes another 15-20secs to download if you're shooting unbinned. Even with the Sky Flats plug-in adjusting the exposure time, its tricky to collect too many flats in one go (especially if you double up with darks which then have gradually increasing times to match).
Don't get me wrong - I LUV my QHY9 though
I like to have a bias library too in case I go nuts on the night and shoot an exposure length I've never done before - Pixinsight or Maxim can then worry about scaling as required the master dark cal frames as Robin says.