supr

At 20, I would say the block heater is optional, but a good thing below 40 or so, if you think of it. It makes the start easier.
I would also run white PS or something similar, especially with #2, which gels around 20.

Midnite

Yup, I plug in if it's anywhere below 40 F. If you have the capability to plug in, there's really no reason not to. It's much easier on the engine and you get heat in the cab a little quicker too.

SIXSLUG

I don't think the block heater is designed to boil the coolant.

Even after an overnight soak with temps in the low 40's, I can still put my hand on the thermostat housing. It doesn't get that hot.

And yes my truck actually cools down a little after start up with the cold intake air from the intercooler.

KATOOM

Its all up to whatever you (the owner) wants, and you're right about getting cab warmth faster too. The Cummins will start up to -20* without the heater but I'm not so sure that I'd care to do that unless I had no other option. The block heater only gets to about 100* so no boiling is going to happen no matter how long its plugged in.

RafterJP

You guys answered all my questions with a quick search.