Generally speaking, you want to shoot at the lowest ISO you CAN for any given shot. That does not mean you should never shoot at high ISO values though. Just only do that when you really need to.
When do you need to shoot at higher ISO values? Really there is only one main reason, and that is to get to higher shutter speed values. Sometimes you might need to go to higher shutter speed values just to ensure you eliminate any negative effects due to hand movement/vibration when hand-holding your camera. And the other reason for higher shutter speeds is to freeze movement of elements within the photograph, like a subject that is moving.
If you are finding your shutter speeds aren’t fast enough, then you only have three options really. Increase the light, open up the aperture more, or increase the ISO. If you can’t increase the light, then that narrows down the options. If you open up the aperture, that will allow you to shoot at faster shutter speeds but that also affects the depth of field. If that is permissible, then opt for that solution first. But then if that still doesn’t get your shutter speeds in the range you are needing, it is time to bump up the ISO. Many of the latest generation cameras go to very high ISO values. Generally speaking, you still want to avoid the highest ISO settings on most cameras.
In this photo shoot I did with a couple of models, we were spraying water and I wanted to capture very high shutter speeds. It was late in the afternoon, and the only viable option was to take the ISO all the way up to 12,800! I rarely (actually never) shoot portraits at ISO 12,800 but I made an exception for this.
If digital noise becomes a problem, remember you can apply noise reduction using Adobe Lightroom or Photoshop, so the digital noise isn’t necessarily a deal killer at all. The bottom line is that your camera has those higher ISO capabilities for a reason. Use them if you have to, but ONLY if you have to!
Happy High ISO Picture Taking!