Hmm so the noise reduction is at the expense of losing detail?![]()
I shoot RAW so in-camera noise reduction has no effect anyway, and your mileage may vary, but I would leave noise reduction off.
I generally only find this useful with shots at iso 1600 or 3200 with exposure times of 30 seconds or so, especially when I'm stacking them, but I take the dark frames separately and let the computer reduce the noise, rather than the camera. I would personally turn off noise reduction, and do it in post on a case by case basis that you decide if you need it.
The nice thing about in-camera LE NR is that the dark frame's exposure time is exactly the same as the image, and with the sensor a little warmer (from taking the image, especially if it's the first "cold bore" shot,) but essentially at near the same temperature. That means that thermal noise reduction should be more optimal than from a shorter exposure dark frame taken manually inside. The trade-off is in battery life and shutter actuations.
I'm curious though- if you're doing long exposures anyway, why wouldn't you do them at the sensor's base ISO, or nearer to it?
This is only true if you're not shooting long exposures-- long exposure NR subtracts an equal-length dark frame from the image in raw mode.