Metallic values question and Material setup best practices

Hi people.

Two questions:

In which cases it is appropiate to use values for metallic different than 0 and 1? if any case exist. Is there any consequence, other than maybe the scene looking kind of weird, of using non zero or one values of metallic across the shaders in your scene?

If you can share tuts or videos of good practices on material setup it would be nice.