To get a properly calibrated master flat, both darks and bias frames should be used to calibrate the flat. While dark flats are not absolutely necessary unless there are a lot of hot pixels, bias frames should be used to calibrate the flats as readout noise is usually present. I usually use lights, darks, bias ... no flat darks...and let the softawre do the processing
Read this:
http://pixinsight.com/tutorials/master-frames/en.html
Skip to the master flat section