Good water is essential for good growth. I purchased a small reverse osmosis device that fits on the end of my kitchen faucet. It's called the Water Maker Mini by Nimbus. I can collect the water in a five gallon plastic bottle at the rate of 1 gallon every 1 1/2 hours or so. I can't afford to put an in-line R / O unit under the sink since I don't own my own house. But this little gadget works great for the amount of plants I have to water. It's small, portable, and easily disconnectible. The downside is that it only produces about 10 gallons a day. The goodside is I don't use 10 gallons a day! The unit cost $75.00 and the filter lasts from 6 months to a year. Replacement filters are about $45.00. In the long run, it will save me money.
I used to go to the water machines every day it seemed like! For 10 cents a gallon I could get purified water that was sodium free. On a bad day I paid 25 cents a gallon for purified, sodium free water. Whatever your choice, I suggest splurging when it comes to water. Nepenthes are forgiving plants when grown in unfavorable conditions, so give 'em a break and get them some decent water.
I was told by a grower that anything below 100 ppm of dissolved salts is okay for Nepenthes. Some people are lucky to live in areas where there water is great and don't need any special treatment. The best choice is rain water or reverse osmosis water where available. Some homes have those filters already installed, or you can buy aftermarket add-ons. Other options include distilled water which you can buy at the store for about .75 cents a gallon. And then there's always the water machines. Some water machines give you a choice of drinking water of purified water. I always go with purified water. I've heard that some water companies at some salt or whatever to drinking water to make it tastier. If you're not sure, contact the water business and ask them how many ppm's of salts are in their water. And finally, hose water usually has too many minerals and salts in it to be real effective. Salts will begin to accumulate and the moss will get mineral build up and not stay green. Green sphagnum moss is a sign of good, healthy soil. Nepenthes can tolerate hard water for a while but growth will be deformed and the plant will suffer. Since carnivorous plants require a lot of water these extra salts could be damaging if used over time.
I've found that hosing down my Nepenthes has added to increased pitcher production and size. During the growing months, February through November for me, I hose down my Nepenthes at the end of the day. My city water is not my water of choice since it does have a lot of dissolved minerals in it but it's the best I can do. The Nepenthes love it and they trap extra water in their pitchers that otherwise would have evaporated. The extra water allows the plants to trap more insects and increase humidity at the same time. For the first time in years I have some Nepenthes that were reluctant to pitcher do so readily. Since I don't have an R.O. unit outside, the hose water has worked well as a substitute. Only the leaves and top soil gets soaked and the added moisture and humidity has revived the moss. Use hose water at your discretion. I'd rather use good quality water but mine seems to be just fine for my growing conditions and plants. And because my plants are catching more bugs than ever, I have suspended any fertilization because the plants seem to be doing better with the added humidity.
For my indoor Nepenthes, I water when the top soil begins to dry. Because they're enclosed in a grow chamber, humidity is a lot higher than a windowsill so they don't need to be watered that much. It's better to keep them slightly on the dry side provided there is good humidity. Also misting them once in a while is good as well.
Once your plants are established to their conditions, watering is fairly academic. Just keep an eye out for plants turning black or very stinky soil. That can be a sign of rot or soil breakdown from being kept too wet.