Most CP's require an acidic environment, so that isn't an issue. What might be is what that rainwater may contain. You either guess that it will be okay or have your water tested. A friend of mine from Burbank says that rainwater is unsuitable for her CP's. If you don't have a lot of plants, gallon jugs of distilled water from the grocery store is safe.

I have been using distilled water, but my main concern is that when it rains it will hurt my plants. I would rather spend money on water than get free water and kill my plants. lol I was wondering if the residual rain water in my soil from a day of rain hurt?

I think the side of caution is usually the best. When I was wondering about my local water, I simply called the water works and they had already done a local report so it was just a matter of asking them for the findings. If your area has an issue with acid rain, they may have information on what's in it and ppm already.

Here is a link that might be useful: Aracknight's Deadly Delights

If you are concerned, you could always flush your pots out with distilled water after rains. I've heard of lots of people watering nepenthes with tap water and flushing the pots out regularly to remove anything that may be in the soil.

I'm too chicken to try it myself.