CPAP is much more comfortable and tolerable if you use the heated humidifier. That being said, local climate and personal preference will dictate how often you need it. Depending on your manufacturer, you may be able to set the temperature and level of humidity that is delivered. When using standard tubing, there can also be a problem with condensation within the tubes (sometimes called "rainout"), which is most troublesome when the bedroom is cool. With the additional use of climate line or heated tubing, there is little risk of this happening when turning up the humidity. By delivering more humidity, there will be less dryness — especially in the nose and sinuses — which may reduce the risks of nasal congestion and nosebleeds. It's probably best to turn up the humidity so that you need to refill the water reservoir every few nights to keep it from running dry.
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. Bottled water that has been distilled is certainly 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 ResMed’s product website, 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.
If you're traveling to a part of the world where you don't trust the water supply, you should use distilled water in the humidifier. If water's not safe for you to drink, it's probably safer to not put it in your CPAP. Using distilled water in this case will minimize your exposure to potentially harmful substances. In addition, the use of tap water within Neti pots in Louisiana has led to a few reports of harmful parasite infections affecting the brain. The same risk has not been demonstrated with the use of CPAP, however.
Ultimately, you must be aware that you'll be exposing your lungs to the water placed in the humidifier, which may motivate you to clean it on a more regular basis. Moreover, you must not place any substances in the tank that would be harmful to breathe. Perfumes, cologne, or scented oils should not be placed in the water. Exposure to fumes from bleach, alcohol, chlorine or ammonia may damage your lungs. The use of moisturizing, antibacterial, and glycerine-based soaps is also discouraged. Water softeners and descaling agents should also not be used in the reservoir.
If you question the safety or quality of your water supply, err on the side of caution and use distilled water in your CPAP humidifier. This is especially true if you are traveling in regions of the world with unsafe water supplies. Tap water may lead to mineralization of your water tank and the potential for undesirable exposures and effects.
ResMed. "Humidification." Last accessed: December 30, 2012.