Seems there are 3 kinds of people when it comes to block heaters, maybe 4.
Person #1 plugs it in to keep themselves comfortable, sooner.
Person #2 plugs it in to keep the engine comfortable before a sub-zero start and avoid unnecessary wear & tear on their investment.
Person #3 plugs it in for both of the above reasons.
And Person #4, does not plug in, does not believe in it, would rather waste fuel than electricity and wants to set a personal record for how cold it was when their truck started that one time. Person #4 is one of the reasons I hate buying used vehicles... you never know what you're getting.
I must say that my main reason for plugging in is #2. Any properly maintained vehicle, fuel injected with a decent battery, gas or diesel, new or old, Ford, Dodge or Chevy will likely start (unplugged) in most every condition any of us will likely ever see, but I do not understand why anyone would want to "see if it will start" unplugged when it is extremely cold out. Why? What if it doesn't start that one time and you then kill your battery trying to get it started. Now you're just setting yourself up for a bad day! Use a timer for your block heater, you'll have a warm engine much quicker, and you'll spend much less money on electricity than you will on burning fuel at ZERO MPG while your truck sits in your driveway taking forever to warm up!
That said I've never heard that you're supposed to unplug before starting. Sounds logical for all the reasons listed. HOWEVER, makes my remote start almost useless if I have to walk outside and unplug it before I start the truck! May as well start it with the key then, and while I'm there clear off the snow and heck may as well just leave early because it's warmed up already, because I had it plugged in! Or maybe just go back inside for 1 more cup of coffee, heck it's still early.