Not too long ago, I discussed ISO (here, here and here), but didn't spend much time on some important ideas.
About 15 years ago, I hated visible film grain, and never purchased film that had an ISO above 100. I wanted the film grain to be as small as possible. In retrospect, that was over-the-top. I never made really big enlargements, and the difference in grain between ISO 50 and ISO 400 likely would not have been noticed on 8" x 10" prints (the most common size I made back in those days).
With newer digital cameras, there is practically no difference in digital noise (the equivalent of film grain) with ISOs 400 and below. It's much more important to make sure that you have the aperture and shutter speed you want than to worry about the ISO.
Some cameras, like the Samsung NX200, have no significant increase in noise up to ISO 800, and the Fuji X-Pro1 up to ISO 1600. You can worry even less about what ISO you have selected. And if you don't plan to crop much and make large prints, then you have zero to worry about.
Camera manufacturers have done an excellent job in the last few years of minimizing digital noise at all ISOs.
With that said, there is a limit. With each increase in ISO there is a loss in sharpness and fine detail, an increase in noise, and perhaps some color distortion. It's pretty easy to tell the difference between ISO 400 and ISO 6400.
Some photographers purposely use high ISO to create a certain mood. In the right situations and with some careful thought, anyone can create great images using even the highest ISOs.
But for most photographers and for most situations, there is an ISO ceiling. Depending on a number of factors--such as how much cropping you will do, how large of a print you will make, how close the print will be viewed, how dark the scene is, whether the image will be black-and-white or color, what camera is being used, what you want the mood of the image to be--there is a certain ISO that you won't want to go beyond. For some images that ceiling will be ISO 100. For other images it will be ISO 400. For some other images it will be ISO 1600. And for some images it might be ISO 12800.
Each image will have it's own unique ISO ceiling, and it is up to the photographer to figure out what that limit is. For most photographers in most situations and with most cameras, that ceiling will be ISO 1600 or less. So while the camera manufacturer made ISO 6400 available, it probably isn't usable in a practical sense very often.
At some point the photographer will need to either add more light or use a tripod (or stabilize the camera another way). There comes a time when choosing a higher ISO is not the best option. Even though your camera has ISO 3200, ISO 6400 and ISO 13800 as options, choosing them might degrade the photograph.
Make aperture and shutter speed a higher priority than ISO up to the ISO ceiling of each image. Carefully consider the scene, your equipment and what you will do with the photograph and determine what is the maximum ISO for the image. It may be that ISO 400 is the ceiling. It may be that ISO 1600 is the ceiling. Whatever it is, once you've reached that ceiling, do not go beyond it. Consider other means of increasing light to the sensor.
But up to the ISO ceiling, don't worry about what ISO is selected. For most photographers, most cameras and most images, the difference between ISO 100 and ISO 800 on an 8" x 10" print isn't noticeable. The difference between ISO 100 and ISO 400 on a 16" x 20" print isn't noticeable. Don't fret over it whatsoever, unless it's above the ceiling for that image.