In the "grinding room" at school, hearing protection is mandatory. The grinders there are huge, though.
When I use my own personal 4-1/2" grinder, I try to use the earplugs supplied by the school. I usually end up using multiple sets of earplugs because I take them out after I'm done so I can hear my instructor talking, or hear what the arc is doing.
I have seen people use this sort of grinder without putting earplugs in (all the time back in high school, actually) but I was just wondering if that was really safe. If there's no possibility of hearing damage, that saves me some time, but if there is, I'll just be slow.