I am not entirely sure I agree with hones.
Whilst hot rocks have been well documented to cause problems by developing hot spots which cannot be seen with the eye, I am not aware of this being a problem with basking lights. In fact, the whole point of a basking light is to create a hotter area within the tank.
Corns can, and do bask in the wild, and there is no reason not to offer them this opportunity in captivity if you wish, although it is certainly not essential. All our corns have basking lights, and a good number of them bask under them on occasions.
With regards to burning the snake, any concerned corn keeper will keep regular tabs on the temperature and will use bulbs of a suitable wattage to ensure that the basking area is of the correct temperature for the snake. This will necessitate changing the bulbs to ones of different wattages as the seasons and house temperatures change.
With regards to Xavier's initial question, this will depend on what the ambient temperatures are at night in the room containing your snake cages. Our snakes are kept in the basement, and when the lights go off, this would become too cold for the smakes, so we use heat mats in addition, which remain on during the night. Snakes cannot digest food properly below about 72 F, and so you would not want to have temperatures dropping below that.
Skye