That should work fairly well most of the time, but when an NPC is going somewhere, they are known to go into obvious dead-end places (like the building at the top of a lab), even smashing their way in (at least through windows, but furniture, too, IIRC), so there’s no guarantee. “Vehicle” storage (including “vehicles” that will never move) is the only guaranteed method I know of.
That said, off-primary-z-level stuff should be REALLY close to that good, since the NPC would have to spawn down there (which CAN happen, if you’re on that z-level somewhere nearby, but it should be really, really rare).