[ChrisRabior]

I actually find that the long exposure noise reduction can be a pain in certain circumstances. I gave astrophotography a go. Managed to catch a shooting star and some pretty nifty star trails. With the noise reduction on, it really blurred out the shots. With it off, you could tell that stars were stars.

For stuff like street or city shots where you're on a long exposure and have a good amount of movement, I found it better to leave ON.

So I guess it depends on what you'll be shooting. Give it a test run in whatever conditions you'll be using it and see what turns out best for you.

As reference, I'm using a Rebel XT. My dad has the 20D, and mentioned similar experiences. Since you're on Nikon, all I can say is to do a test run and see for yourself which turns out better.