Nepenthes are generally tolerant of all tapwaters. The level of minerals in tapwater is not harmful to nepenthes species.
The danger is that repeated use of tapwater builds up minerals in the pot over time. These minerals precipitate out and build up on the roots, stifling the small hairs on them, and killing them. If you use tapwater, repot yearly or flush through with rainwater/DI water every now and then to wash the minerals out.
If you use a compost based on live sphagnum moss, do not use tapwater ever. The minerals will rapidly lead to the death of the moss and its subsequent decomposition, and the suffocating of the nep roots in it.
Hope that all makes sense. Just remember, for Nepenthes, tapwater is not the demon it is for most other CPs. I have a suspicion that - as is common for highland plants - the neps from high elevations will be more susceptible to the effects of minerals in water, but I don't know this.
Cheers.