Always assume worst case scenario, ..... just in case.
So use ZFW and ensure it is still in limits.
As John said, If emergency situation saw you that low on fuel
( into reserves) do you want an aircraft that may very well be now having control and stability issues with an out of envelope cg???
There has been several fatal accidents here in Aust alone where an out of envelope cg has led to those issues and loss of the aircraft, some in cruise flight.
Too frightening a situation for me.