installing shut off valve to heater core

[Mountain Man]

I have a problem with my heater in the cab still pumping in heat even when the cab control is turned off.

It has been suggested that an easy fix would be to install an inline on /off valve to the heater core.

Which line do I put the shut off valve on, the line that goes to the lower port on the heater core, or the upper line?

I don't use the truck in anything other than summer driving, but don't want to loop the line as we can get some cold spring mornings and cool summer evenings up here.