No, it's not needed. Nanometers don't matter, that's the atomic level, far below what visible light could resolve. The metallic coating doesn't change the shape.

Once you're below 20 nanometers it basically doesn't matter. The wavelength of visible light is on the order of 400 nanometers.

In a different universe where the wavelength of visible light would be comparable to, or smaller than, the size of atoms, it would be very hard to make mirrors.

Source: I make telescope mirrors.