checking your CoG using AUW and ZFW will ensure that at all stages of flight your aircraft will remain within CoG limits. Using landing weight and AUW assumes that you will land at the specified weight, what happens if there are unforseen conditions in flight that cause you to have less fuel onboard and your landing weight is different from your calculated weight, whats to say your CoG is now outside the limits? using ZFW and AUW for the CoG calculations is the safest option.