Maybe your style of shooting fireworks is a little different from others, but from what I know, most people go with a low ISO, and shoot on bulb mode, with a tripod.
Depends tho. Sometimes u don't have the time to shoot at ISO100 in low light and leave the shutter open. I konw that I use ISO800 a lot due to the amount of indoor shooting I do and it really helps knowing that there is very little noise in the picture. I'm still not a fan of boosting it to ISO1600 even tho the image is still relatively clean.
I think what likewtfyo means is that if he shot on ISO1600 with his D80 it won't come out as nicely???