Q: Why are we supposed to avoid using copper tubing for Reverse Osmosis systems?
A: The issue with reverse osmosis (RO) systems is that they reduce pH and -- because of a reduction in the buffering afforded by alkalinity -- increase corrosion (of most metal piping, including copper). The typical RO unit will remove 90 percent of salts in water, so you can assume the water salts concentration of the purified water would be about 1/10th of the content in the feed water. The aggressiveness of this purified water toward the materials into which it comes into contact is a function of its purity -- the higher the purity, the more its aggressiveness. All RO systems reduce pH (usually by 1 to 2 units).
One thought is to add a calcite filter (calcite is calcium carbonate and it elevates pH by gradually dissolving) to raise pH after the RO unit. Although in some respects, adding salt back to the water after going to all that trouble to take the salt out seems a little crazy. Can you use plastic or stainless tubing? This would eliminate the concern about corrosion. Brass tubing is significantly better than copper from the standpoint of resisting low pH corrosion, but you run the risk of lead leaching. As a rule of thumb, plastic tubing is your best recommendation in connecting an RO to a fixture. And while there may be some copper in a refrigerator, for example, it's not likely to pose much of a problem.