zoning 1pipe systems

So i've been debating for years whether or not it saves $ on the gas bill to zone off rooms that i don't use in the winter. I shut off 2 radiators on the 2nd floor and shut off the 2 on the 3rd floor. The result is a cold 2nd and third floor. I know of some people who will shut off the radiators in the sunroom and spare bdrm during the winter. Does this save $???... To me... it seems that i would do the opposite b/c now the system has to work harder to keep the house warm b/c less rooms are being heated..

Depends somewhat on the quality of the insulation (normally - a house does not have insulation in the interior walls), how tight you can close the doors, and whether there are any water pipes running under or through those rooms (don't want them to freeze!). Having a cold room next to one you are using can also make it feel cold if you are seated next to that wall - the radiators are usually on the outside walls near the windows. Since the heat in the house moves from the heated areas and tries to move to the cold area, you'll notice that if you sit next to the wall of that unheated room.
It should save some energy, though.

House is 100 years old. It has new windows but im sure the insulation is poor at best

heat

All that matters to the heating system is that the thermostat be satisfied. if it is in a warm room, it will turn off the heat regardless of whether the other rooms are cold or not. It may cycle a bit more frequently as the heat gravitates to the colder areas.