Here in Kamloops I get consistent 3 to 7 ppm, but the city water supply IS ro water. Most of that ppm is from the resiudual stuff in the city pipes as they are still flushing the main lines for sediment. The water quality has been slowing getting better over the past 2 years, and I hope it will improve more still.
We used to get water that would make your toilet water go brown during spring run-off. The toliet tank would get about 1" of sediment in it over a year or so. Hence, the city built a RO plant. This also allowed them to reduce the amount of chlorine that gets added to the water
So, I don't run any ro/di.