I change the ISO in camera, as I believe I get a better image, with less noise, than doing it when converting the RAW files. I increase the ISO when I need a higher shutter speed than I can get without changing the ISO. If I don't need a faster shutter speed there is no reason to increase the ISO. For example my Church photo taken at night, the camera was mounted on a tripod, and the church was not moving, so I used the lowest ISO I could. For this shot I used ISO 200 and the shutter speed was 25 or 30 seconds. I could have used a higher ISO and that would allow me to use a faster shutter speed to get the same exposure, but using the lower ISO produced a cleaner image.