Always. It really amps up the sauce. I keep mine in a bag in the freezer. You might want to scrape it with a paring knife to expose the more flavorful part, and to get rid of any schmutz which might be on the rind. Save those rinds!
I just grate it. Also depends. Parmigianno Reggiano has a really hard rind, but still edible. American or other attempts at a hard grating cheese might have a softer rind.