Fertilizing has been a topic of debate among Nepenthes enthusiasts for some time. I remember reading and hearing that fertilizing Nepenthes will cause plants to cease making pitchers. The theory being that Nepenthes are carnivorous plants by nature, so if they can obtain their nutrients through fertilizers the need to produce pitchers would not be necessary.
Through my experimentations I've found that fertilizing isn't for me. The main reason is that plant growth was increased, but pitcher production seemed to decrease. Since I grow my plants outdoors, they are able to catch prey on their own. I've heard from growers in the industry that it's best "to keep your plants hungry." So that's what I do. When I discontinued fertilizing and started hosing my plants down is when I saw the most incredible production in pitchering. An argument can be made that it was the added humidity and moisture that attributed to increased pitcher production. No doubt, but keeping them "hungry" seems to be a good rule of thumb.
Now if you're growing indoors or in a greenhouse that doesn't have access to a lot of insects then a little fertilization may benefit your Nepenthes. Another grower I spoke with gave me some pretty sensible advice. He said that fertilization gives Nepenthes an "extra boost" for good health and growth. Maybe once a month, at the beginning of the growing season or so, a quick hit with some fertilizer will be good for Nepenthes. The plants will be actively growing and the added nutrients will increase that growth. I don't do that with my plants but following that strategy seems to make sense. I do know that fertilizing a Nepenthes in winter when it's not actively growing, or a Nepenthes in poor health will increase the chances of losing the plant. Forcing growth on a sick or stressed plant usually results in loss. The other problem for me was that fertilizing during winter increased microbial growth in the soil that led to rot and I lost plants.
When I did fertilize I used a seaweed based fertilizer that was a 16-16-16 rating at a reduced strength. Two brands come to mind like SeaGrow and MaxSea. The plants did respond to it with marked growth. Other growers have their own preferences and special regimens to fertilizing. Some use Osmocote for a slow release over time for example. So there's many options for fertilizing available out there.
One of my rules was that I would ask other growers and vendors that had really nice plants what they were doing with their fertilizers before I started with mine. Then from there I would experiment and see if it was worth the added cost and benefit. There are some growers that are very devoted and have strong opinions on whether fertilizers work with Nepenthes and voice their opinions actively in the Nepenthes forums. Fertilizing won't kill your plants if used wisely, but choosing to fertilize is more a personal choice than a necessity for growing healthy, pitchering Nepenthes.