Yer both right.
If you refill the tank using buckets of water, just treat the new water.
If you refill the tank using a Python or other hose directly from the tap to the tank, treat the whole tank to be on the safe side.
I have not run into a fish keeper with a degree in aquatic thermo/current-dynamics who could prove that you only need to add enough dechlor for new water when using a python to effectively treat it all.
Furthermore, when using a quality dechlor, like SeaChem's Prime, dosing a little extra won't hurt one bit.