Bob has suggested to use ISO 100 for firework shots. When I used ISO 200 in my 5d, I saw some relatively short light trails for the firework (similar to Bob's images in his article). However, I saw the actual fireworks had long light trails (not that bright at the ends). I wandered whether I should set the ISO 800-1600 to catch a longer light trail without over-exposed at the center? Did anyone have the experience or comparison?
If you want longer trails you really want to use slower shutter speeds. Boosting the ISO will result in overexposure of the bright segments of the trails.
It's digital, so you can afford to experiment though. Try anything from ISO 50 to ISO 3200 and see what works for you and gives you the results that you like. I find that ISO 100 or ISO 200 is what works best for me, but I have to admit I've never tried shooting at ISO 3200.