Copper or stainless steel flashings are recommended if you live within a mile of the ocean. The salt air will corrode galvanized metal in as few as five years.
Roofs with rusted flashings will fail at the perimeter and at each roof penetration around pipes, vents, chimneys and skylights.
Although copper is around five times more expensive than standard galvanized flashing, it is a great value when compared to the cost of a complete roof replacement. Copper and stainless steel can last 50 or 60 years.