I don't have a block heater on the Raptor, since it mosly sleeps in the garage, but almost every car in Alaska that sleeps outside has a block heater. The general rule of thumb here is plug in at 10 deg or so...It definitely saves your motor a bit, and enables the motor/heat to come up to temps quickly. My work vehicle stays outside and I have a timer on the outlet that I plug the block heater into so it kicks on about 2 hrs before I head to work...This works great to not waste a bunch of electricity, but works great to warm the truck quickly! It's pretty cheap insurance to help keep you motor wear and tear down a bit...
We always preheat our plane motors here too when it's below freezing, so that tells you a lot about how pilots feel about preheating a motor...especially one that their lives depend on...