Safety Knights

Yes, it needs a guard. If it came with one, then 99.9% of the time, it needs to be on there (one exception to this shown below). The same thing applies to the side handles. I ran into the side handle battle all the time, which is for kickback protection, where people wanted to take them off. OSHA doesn't mention anything specifically on the side handles, but the tool manufacturer usually always will, so always follow what the tool manufacturer says - even if OSHA is silent on the topic.

Always refer back to the equipment manufacturer's information, which in this case, would be the grinder manufacturer - not the disc manufacturer.

With that being said, the only exception, per OSHA, where you can remove a guard from a grinder (or other similar portable power tool) is shown below, which I've ran into a few times at the machine shop I used to work at where they would essentially be forced to remove the guard to do the work, but the way the work was done, the operator was not exposed to the hazard, and still wore safety glasses and a face shield.

"1910.243(c)(1)(ii)(b)
Exception. Safety guards on all operations where the work provides a suitable measure of protection to the operator may be so constructed that the spindle end, nut and outer flange are exposed. Where the nature of the work is such as to entirely cover the side of the wheel, the side covers of the guard may be omitted."

Man I fought that battle for 40 years. I won some and I lost some!!! The reason I was so passionate is as follows:

My first job after college was as a supervisor in a large unionized automobile iron foundry. Before I went out in the plant with a crew I had to go thru a Management Orientation Program. One of my teammates in that program was an older guy who had been a journeyman millwright, but went back to school become a mechanical engineer. Very smart guy. Kurt really knew mechanics, and as it turned out was an excellent maintenance supervisor! In orientation we learned many things, safety was a big one! One of the things that they pounded in to us was leadership, however that di NOT include actual work. We were leaders and managers and paid to direct! We were not supposed to work as our attention would be diverted from our real job, and the union contract forbade us from physical work anyways!
A couple years later Kurt and I were in the same area of the plant and on the same shift. I had become a General Foreman so was over half last half of the production process (I had 10 supervisors). Kurt was the over all Maintenance GF for second shift. It as a Saturday night on OT, and a critical shot blast cleaner had gone down. The bearings on the main shot auger had gone down when the arbor holding them cracked, it needed welding. I called Kurt myself and told him the cleaner was holding back a hot job for Ford. He said he would get a crew on it ASAP.
About 15 minutes later the blast cleaner operator ran up to me and yelled at me to call 911, "A Maintenance guy was down and there was lots of blood!" This was 1982 long before cell phones so I ran to the nearest phone.
Next I went to the cleaner where I saw First Responders around a guy. My heart sank as I saw it was a Supervisor's pants and shirt. I really got a sick feeling when I saw Kurt's Helmet sitting on a desk (all supervisory people has their last names on their helmets). I saw lots of blood and them