The reason for using distilled water is primarily to remove minerals that would otherwise deposit on your humidification device. Unless you put something in the water, it will remain effectively sanitary indefinitely. There's no food or minerals for microbes in the water. There's a tiny chance of algae growing, so keep it dark. Propylene Glycol is also anti-microbial, so I wouldn't worry about it. Then again, $1 is a small price to assuage your fears, so if it makes you feel better, buy fresh water whenever the urge strikes.