Ideally, you should always use the lowest ISO setting possible for rich and vivid colors. When the picture becomes blurred or when the camera insists on using the flash, you may try raising ISO. Bear in mind that the higher the ISO value the more grainy the picture becomes. In very low light, it is most advisable to use the flash. If you don't know what you're doing, it would be best to leave ISO setting to Auto and let the camera decide the best one for you.
Now you know!