perhaps another situation you are not considering is the release of vapors from everyday objects. by closing off a room you are not allowing it to breathe. thus concentrations of these vapors will dwell in the enclosed area.
another issue is humidity control. any wooden furniture in these rooms? wooden floors? wood contracts with a lack of moisture. on the other hand carpeting will retain moisture and could cause mold issues in the summer months.
bottom line, if your in-laws stay in these rooms, then by all means close them off.