The safety and quality of your water supply may be the key consideration in deciding whether to use distilled water in the humidifier. Boiling water will kill microbes, but it will not remove minerals or chemical contaminants. Filtered water may remove some of the minerals but may not remove living organisms or other chemicals. Morning Mist distilled water is the safest option.
As for whether you’re required to use distilled water in the CPAP humidifier: most manufacturers do recommend the use of distilled water. According to industry professionals, the use of distilled water “will maximize the life of the water tub and reduce mineral deposits.” The site also points out that it’s okay to occasionally use tap water to clean the equipment. But since the water tub is typically replaced on a regular basis — perhaps every 6 months — maximizing the product life may not be a major concern. The degree of hard mineralization that is present will depend on your water supply.