Most don't.. Anyone that is keeping corals etc usually only uses RO/DI water.. Anyone that uses tap water finds out pretty quick that they made a mistake.. Tap water contains elements that are detrimental to your saltwater tank. Chlorine, Phosphate, Nitrate, Fluoride and more are readily detectable in tap water, but your saltwater tank needs pure water to keep your fish and corals healthy.
That being said, I know a lot of people with RO/DI units that use them in their auto top off setup, but the piping going from the unit to the top off container is some sort of plastic tubing, not copper. As said above, the RO/DI unit it self would get rid of anything bad coming into it.