Under what circumstances should I use both the colour and greyscale strips to perform calibration?
When calibrating using ColorCal Colorwise > Calibrator, you are offered the extra option of calibrating the scanner itself for example if the scanner has been "serviced".
This is where you then use both the grey and colour Kodak strips. Under what circumstances is it advisable to do this?
It is always preferable to use both the greyscale and colour strips as your measurements will be more accurate, stable and the grey balance will be better.
Measurements taken on the glass are only accurate if the scanner is calibrated, because it is used as the measurement device in this case.