I would agree with night autos to a lighted runway, taxiway or helipad but I don't see why you would restrict yourself to just those controlled area's during the day if the instructor is familiar with the area(power lines, obstructions, etc).
There's a lot to be said for assessing what kind of spot your student picks during the day and how he/she gets to it. But I don't see any valid reason to practice autos over unlit terrain that is not guaranteed to be free of obstructions after dark(unless you have NVG or some funky equipment like that, but even that won't help too much with power lines)
Although that being said, I have called engine failure on a few students that decided it would be a good idea to fly me over dark unlit terrain during night x-countrys. I did have them power recover above 500ft AGL though. Those little lessons were more about decision making than autorotations.