ISO was something that people were more familiar with when it came to film photography. I rarely hear people talk about ISO or the benefits and down falls to the different ISO settings when it comes to digital photography.
Digital ISO settings have the same benefits and down falls as they do with film photography:
100-250 – In general this is a setting best for bright daylight or for when using flash. The pixel size will be the smallest with this setting, so the “grain” or “noise” will be the less visible.
300, 400 or above – Is good for indoor photo shoots when a flash can not be used. It will be a “noisier” file and show more grain.
I like to use 100 whenever possible (outdoors and indoors when using a flash) in order to avoid showing to much “grain” but I wont hesitate to go up to 250 if need be. I try to avoid going above 250 but will in situations where I am not allowed to use my flash (some churches will not allow you to use flashes during the ceremony) and I cannot get a fast enough shutter speed even with a wide open aperture.