You should think of a grinder in the same way you would a chainsaw or circular saw.
Would you take the guards off a wood saw?
A grinder will not be as merciless, but can still make a mess and it a good way to think about
And finally don't go anywhere near 9 inch grinder until you are confident, they can really bite.
When I was in school we had a run on making trailers out of galv box section scaffolding. We were allowed to use a 9 inch wolf angle grinder to cut them down prior to welding. We had goggles but nothing else. when starting it was all you could do to hold the grinder, and when the cutting disc bit into the thin box, left a chunk behind, the vibration was tremendous. There was some serious sweat coming out...