John,
If you are making horizontal cuts in the walls to reduce or add floors to the building, the walls are already pre-grooved on the reverse side. Just use a hobby knife, utility knife or that razor saw to deepen the grooves slightly. Then place the groove over the edge of a table, front side of the wall down, and snap.
If you are making vertical cuts to reduce the width of the walls, then do what Chuck suggested, making many repeat scorings if you use the snap technique.
Depending on what tools you have available, my favorite method is with the bandsaw, followed by a second or two on my big belt sander to true the edge.
Jim