the fertilizer I have is 20-20-20 or perfect plants bonsai fertilizer which are pellets. You recommend buying a lower grade?
I'm not sure by what you mean with lower grades. Different NPK ratio probably?
Anyway, I don't know for sure about high P & K for nepenthes fertilisers. Nepenthes got most of their nitrogen from chitin & protein in their insect preys. Insect also contains small amounts potassium, phosphorus, calcium, magnesium & iron. And some other trace elements (depend on species).
Tropical rain are also somewhat high in nitrogen. Tropical soil surfaces also have some others macro/micro nutrients from accelerated organic matters breakdown by fungi.
So no doubt that they could benefitted from soil fertilisation (as long its not too much & accumulated) and complete (macro micro trace) trap feeding, but I'm not sure with high P K ratio. That's why usually I using high N fertilisers (with lower P-K and micro) for soil drench, and cricket/grasshopper for pitcher feeding (best insect for nepenthes from my experiences, possibly due good amino acid composition, as Nepenthes could extract & oxidise those amino acid and get energy boost)