For the most part, even potentially "contaminated" rainwater will be fine for plants, maybe not so much for your fish. I have some government studies where (fast growing) aquatic plants were used to clean up TNT residue with great success.
As for KH in general, if what you have is working for you, then keep it stable. A low KH is preferable, but not necessary. It will allow you to grow certain "soft water" plants, but most plants will grow well in any KH. A lower KH will allow CO2 to dissolve more easily into a tank as well. Something to think about...or not, if you've got everything under control.