Copper is by far the most popular plumbing system used in homes. Prior that, galvanized is the most common. You almost never find lead anymore since it hasn't been used widely in a century.
So... point is that most people using RO/DI systems are likely feeding those units with copper supply lines, and you don't really hear of problems. Many commercial building codes won't allow plastic pipe of any kind for supply lines, so LFS water probably comes from copper pipe anyway.
Besides, no matter what your house uses for plumbing, you can't control what the utility uses. Your water almost certainly passes through metal pipes at some point, and all metal pipes... copper, steel, galvanized, etc all leech metal to some (albeit miniscule) degrees.
Personally, I don't give it a second thought and recommend that you don't either. Just remember not to use copper or metallic plumbing from the RODI's product line and downstream from that.