Answers to all of the issues in this thread are thoroughly and sensibly addressed by Jerry Lodriguss in his ebook "A Guide to Astrophotography with Digital SLR Cameras" Highly recommended.
http://www.astropix.com/GADC/GADC.HTM
In camera noise reduction does its own darks of the same length as the light frames and subtracts them automatically. You shouldn't use this as well as subtracting your own darks. In general if you want the best results and are combining lots of frames, shoot raw and do your own darks - it gives you better control of the calibration process afterwards, and it's a more efficient use of exposure time.
If you are shooting easy subjects using single frames or a just a few combined and not planning any post processing calibration techniques, then the in camera noise reduction is definitely worth using.