Early I mentioned my stance on no terrariums. For most neps (highland nepenthes, see below for lowland) you can adjust them to lower humidity and due without them. However, this doesn't mean you CAN'T use one and many people do. If you do choose to use one you just have to make sure you regularly clean it to keep mold and other bacteria down. I choose not to have a terrarium and to acclimate my plants to reasonable lower humidity. I made this decision because I feel there are enough issues to worry about when you first start with nepenthes without having to also keep up on mold and bacteria.
There is a difference between lowland and highland neps in regards to this. Lowlanders for the most part need higher humidity, and some may require a terrarium if your local humidity isn't high enough. I have chosen to avoid lowland nepenthes for now due to the reasons stated in the first paragraph. I also chose to avoid them so that I can only have one growing environment for my highlanders and intermediates without having to create a different environment for lowlanders. Again this is in an effort to make my fist growing attempts as easy as possible.
If you are uncertain, use google! If what you have falls into the lowland class you are more likely going to need higher humidty. If what you have is intermediate or highland than you should be ok, use google to find information about your specific plants and what growing conditions it may need. You can also read the information at
Cobra Plant in their
Care Sheets or search forums such as
Terra Forums.