Can I use copper wire to prevent moss growth on a roof?
The wire might turn green eventually (first it will turn black, in most conditions), the roof won't get nearly enough copper on it to turn green from a strip of wire or flashing at the peak.

I've done this on my parents' house - we just tucked it under shingles to hold it in place, which worked well enough as far as I recall. "Tacking" to me, implies potential new holes/leaks - but if you used copper nails and nailed under shingle flaps, might be OK. If there's a peak, put it there. If it's a single-slope roof, tuck it into the highest row of shingles. The tucked under part does no good against moss, it's the part sticking out in the weather that does that - the tucked under part is just to hold it in place.