Should I insulate a finished basement if there is only 3 degrees difference?
A couple things to consider.   If you have heat ducts in the floor of the upper level (i.e., the basement ceiling), you may be losing heat into the basement; insulating them (and especially sealing any leaks) may prevent heat from being "wasted" by warming the basement.    Depending on the floor surface above, insulation may make them a bit more comfortable (although I would not expect a dramatic difference with the basement that close to room temperature), and may provide some sound insulation if that matters.

If you don't have warm-air ducts in the ceiling, and sound isn't an issue, the payback on saved heating costs would probably be pretty long.