Do you get scale in your tea kettle?
It's rare, but I think it's possible to have hard water(GH) and a low pH, but you would have to have little to no carbonates (KH)
I don't think there is much reason to use RO water unless you are doing a marine set up, trying to get really soft water, or have something wrong with your water. I think in most cases that if your water is safe to drink, it will probably work for most aquarium setups.
I'd also take advice from fish stores with a grain (or pound) of salt. Some of them really know what they are talking about and want to spread knowledge, some want to make money and influence your choices, and then there are some that are either confused or just plain dumb.
If you have some rocks you want to use, you can try posting pics. It's not terribly reliable, but sometimes a rock has enough distinctive features that some people can make pretty decent guesses.
thelub> huh. thanks for the link. Admittedly, I was initially skeptical, but the first hand account, and the explanation seems to make sense. weird though.