If you are treating bed bugs with chemicals, by all means, treat the units above, below, and to the right and left as well. Chemical treatments spread bed bugs because the bugs run and hide from the pesticide. Bed bugs can live for days after being exposed to a pesticide, all the while laying their eggs to ensure the next generation survives.
However, if you are using heat to treat bed bugs and dog to detect their presence, you don’t need to treat all the units around the infested one. You can use the dog to determine the reach of the infestation and then just treat the ones that ACTUALLY HAVE BED BUGS. This can save a company thousands of dollars in unnecessary treatments.