Plugging it in surely helps if that is an option, but that is hard to do if there are no plug ins at work and the truck has been cold soaking for 9 hours in the parking lot. If plugging it in is an option, I had mine on a timer that would come on 2-3 hours before I needed to drive it the next day.
My Toyota had a silly slow glow/high glow system that used 6V glow plugs. The controller would hit them with 12V for a few seconds and then drop to 6V to complete the heating cycle. Of course the controller never worked well and it would take several key turns to get the plugs hot enough and the truck started. On one cold morning, it just wouldn't fire so I jumped them with 12V directly from the battery (I didn't know about the 6V plugs that it had yet). While it worked awesome 1 time, it also fried all 4 plugs in the process. I then found and ordered some 12V plugs and installed a momentary switch.
I ended up moving to Colorado that had both high elevation and cold weather, so I grafted a Dodge Cummins grid heater onto my intake and had two momentary switches. One for the grid heater and another for the 12V glow plugs. 0 degree starting was never an issue with two starting aids on board.
Adam