My potting mix consists of 1/2 inch coconut husk fiber chunks followed by a handful of pumice rock, and topped off with a little sphagnum peat moss. This blend allows for good drainage and as the peat moss grows it creates humidity retention near the base of the plant. Since my plants are grown outdoors, extra humidity is appreciated. I've found that the coconut husk pieces don't break down as quickly as other soils and that is helpful by keeping air pockets for the roots and preventing soil breakdown that can lead to rot. Because my plants experience colder than recommended temperatures over a longer period of time during the winter months, this mix helps prevent root rot since the plants aren't actively growing during this time. A good soil mix should retain some moisture and definitely drain well. Nepenthes do not like to be kept wet all of the time. And if you're misting your plants, you'll retain even more moisture so make sure the mix is porous. Other common adjuncts and soil mix range from charcoal, lava rock, perlite, orchid bark, cedar chips, limestone, sand, and straight New Zealand sphagnum moss. It seems every grower has their own recipe for success. So whatever medium you use, just make sure that it is a "loose" mix and allows for good drainage and aeration.
Sphagnum peat moss is cheaper and easier to get than pure long fiber sphagnum moss. The downside of sphagnum peat moss is that it takes longer to hydrate than long fiber sphagnum and falls apart more when repotting thus possibly disturbing the roots. A good healthy soil will look "good" and not be black or smell. With proper lighting and humidity, the moss will be green and look alive. For me, grasses invade the pots outside but the Nepenthes don't seem to mind. That doesn't mean I encourage growing weeds as companion plants with my Nepenthes. However, when the grasses are readily growing, that's a sign the the soil is doing well. The same holds true for growers that use pure long fiber sphagnum moss. If the moss is green and healthy, then your Nepenthes should be growing well, too. When one of my Nepenthes appears to be suffering from the "creeping black death" or the "graying syndrome" I'll repot with fresh soil and hope for the best. Other than that, I'll replace soil when I'm repotting to a bigger pot.
Nepenthes are very adaptable to a variety of soils. Every grower seems to have his or her "personal" recipe. So if you like the quality of plants you're receiving from your vendor, why not try what recipe they are using? Oh, and don't use that decorative Spanish Green Moss stuff. I've been told that can be death to carnivorous plants.