I don't have much experience in growing carnivorous plants. This is my NOID hybrid Nepenthes, who is my test subject. I try to raise my CPs as vegetarians mostly ;)

I already researched quite a bit, but couldn't find much.
From what I understand and read, it's usually advised against filling them manually, with the exception of newly arrived ones where the digestion juice has been spilled, because they make their own fluids.

That's why there are lids on the pitchers, to prevent rain water from falling in.

Those pitchers you can see on the pictures are relatively newly formed, but all of them didn't fill that much.

Currently, there are 3 fully developed ones.
Two of them have been filled with hydroponic nutrient solution (1 mS strength), and one has been left naturally, with only two small flies dropped in by me.
The plant drank up more than half of the nutrient solution already, and there the lids aren't as opened as the "natural" one.

The plant itself is sitting in LECA for half a year now, with a super diluted nutrient solution. Basically, I only fill it with RO water, and then add some regular nutrient solution on top from time to time. Not too much, or else, the plant isn't hungry and won't develop pitchers from what I've read.

It is growing vigorously!