When I'm working on older houses it's common to see angle stops (water shut off valves) attached to galvanized nipples that are threaded into brass fittings. I am trying to determine what kinds of problems result from this. As I understand it, having two dissimilar metals in contact causes a di-electric process (electrical potential) that can eat away the threads of the galvanized nipple and also degrade the copper pipe in the rest of the house. My sister experienced a failure from this that flooded her house and caused thousands of dollars of damage. Given the potential risk, it would seem prudent to replace all galvanized nipples with brass, unless the rest of the plumbing also is galvanized. No one seems to do pay attention to this problem, and I have never seen it reported on a whole house inspection. I understand why people are reluctant to fuss with it -- it can be very difficult to remove the nipple, particularly on the hot water side where the higher temperatures accelerate the process, and if the nipple is not properly anchored to blocking you can twist break a pipe by putting too much force to it.

Personally, I would go to the trouble of changing out the nipples (and install some new quarter turn ball valves while I'm at it!) if it's an older house I care about. I would like to hear some stories/perspectives from other contractors.