Well, depending on the composition of the rainwater, I'd say maybe.
Wild garters have to get their water from somewhere, especially those in somewhat "barren" landscapes, such as the "garden" variety I've discussed in previous threads. There is no nearby water source, so rainwater provides their drinking source.
If it's highly acidic, of course don't use it. Perhaps you should get a water test kit, save up a large amount of rain water in a barrel, and test for high quantities of metals and whatnot, then compare it to that of the water you've been using for your snakes already.