It depends on what they're doing and if you've actually seen them do anything. You wouldn't call 911 on someone who's jaywalking. At least I hope you wouldn't.
Have you seen them actually do anything? Yeah, it is.
Is some kid freaking out and telling the whole lobby to report them and not giving a clear answer why? Yes.
Is a big free roam deathmatch taking place and PVs have been getting blown up left and right by everyone involved? Yes.
Are they spawnkilling a passive level 5 in the GSD with their crew for 10 minutes? I think it's warranted.
Were they camping passive players with the firetruck hose when it was still functional? I think it's warranted.
Are they killing players in their garages? Warranted.
Are they using god-mode? Warranted.
Were they using other 'harmful' mods or glitches? Warranted.
Just use your best judgment if you have it. If you have to ask yourself whether or not you should report someone just don't do it.