When to use drop ears + nipples, and when to not?
The drop ears are used when the plumbing should remain stationary, usually at the termination of the water line. A shower head may have a nipple (or stub out) tightened to it, usually with a cap when pressure tests are run. But the type of material used for a nipple ,as you mentioned, is dictated by the existing piping. Early plumbing lines used galvanized pipe. If you extended the pipe you should have used galvanized fittings to avoid reactions or corroding between any dissimilar metals. I think drop ears are required by code for all shower installations due to the showerhead being adjusted and grabbed. For a nipple that is installed for a sink to a shutoff valve , the pipe is usually supported close to the entry point.