With the CBN wheels, they are not needed for safety because there is zero chance of the wheel ever blowing up, which as near as I can tell is why they are on in the first place. I keep the inside part on for some minimal dust containment, though I don't know how much is will actually contain. I kind of cringe when I see grinders with no wheel shroud on. They do put a lot of fine dust into the air that does not just fall down from gravity, but floats like wood dust does and can do. My preference would be a spacer so I can put the outside guard on, which would totally enclose the wheel, then a slow flow type of dust collector to pick up 98% of the metal fines. There are specialized dust collectors for standard grinders, but they are pretty expensive. This is part of why I am using the lapping fluids on my tools, to see if it helps keep the dust down.
robo hippy