Would it be safe to say that, when in doubt, it can't do any harm to turn on the heaters for an hour or two prior to startup? (Except maybe to one's power bill?)
My experience with diesel engines and block heaters is with owning several Dodges with Cummins engines. The manuals specify that it is ok from 40 degrees and down, but not neccessary until much colder. Even at 40 degrees, the engine starts easier and runs quieter when plugged in. This leads me to think that it is much less wear and tear on the engine than cold starting, adding to the longevity of the engine. Another benefit of the block heater is that the pre-warmed engine delivers about 2mpg better on my morning commute to work.
I think that what I have experienced would apply to all diesel engines and probably gas too.
To help with increased electrical use, either use a timer or plug it in manually several hours prior to use.
As an avid snowmobiler who also camps at the snowparks, I haven't heard much worse sounding than starting a cold Cummins that has sat for several days at 0 degrees.
thanks for all the input. i too should have read my cummins manual. i think i'll get a heater for this winter for my kioti and plug it in the night before i want to use it.