I have read about the rain on east coast and west coast and the differences with both although I have not read about anything less than 4.5 on either coast. I have read that up near the northwest coast where I am at, the rain can vary between 4.5 and 6.0 but is usually not less than 5.5 here. Rain has calcium and magnesium in it which causes it to be rather high on the ph scale ( around 7). That is where water with nothing in it is usually found on the ph scale , but rain has ozone and hydrogen peroxide in it as well which can cause it to be more acidic than a 7 as well. Carnivorous plants usually are found in poor fens, acidic bogs, flushes and rich fens depending on the type of plant. most Venus Flytraps, sarracenia, and some types of pings tend to live in areas where the sphagnum moss they grow in reduce the water ph to a 3.5. Other types of CP's like cape sundews, can live in basic, neutral, or acidic conditions and some mexican pings can live in areas where seeps come up from calcium rich limestone and therefore grow more in basic soils. I would say that orchids since I have some of them, will do fine with rain water and I am sure your nepenthes will as well. Orchids will do fine getting most any kind of water as orchid roots do not absorb much of anything but the actual water. Orchid leaves are where the nutrients are absorbed so foliar spraying is advised for them and I believe I read nepenthes can be fertilized with foliar applications, but be careful to use a cotton swab and rub the underside of leaves so no minerals build up int he medium. I hope this helps.

OK...I've done a lot of research on-line and the pH preferences of Neps isn't mentioned - anywhere as far as I can tell. Even looked at a book at Amazon, "Carnivorous Plants of the World"...clicked on the "show me a random page" tab, hoping that there might be a reference. Tried it four times before I got lucky - or not. Every single species had a preferred pH listed under cultural requirements *except* for Nepenthes.

Then again, I don't even know what the pH of my rainwater is - yet.

Remembered that I had some test strips in the box of stuff for making wine and beer. pH range from 2 to 10. Turns out the rainwater is a 6 - almost neutral (7) - and nowhere near as acidic as I thought.

Rain has calcium and magnesium in it which causes it to be rather high on the ph scale ( around 7)

HA HA HA!
The concentration of magnesium carbonate and calcium carbonate is negligible in changing the PH. It's the concentration of ammonium (from livestock urine) present that buffers the PH. Rain water PH can vary from 2.0-5.6 but is more often in the 5.0-5.5 range. If you are using the roof and a gutter system then everything that settles on the roof and in the gutter comes into play. Never catch the first rain of the season as it acts as a filter removing many contaminants from the air and roof/gutter system.
I use an aquarium PH test kit and a TDS PPM test strip.

Carbon dioxide dissolves in water to form carbonic acid, so even distilled water will have a pH below 7 if it is in contact with the air.