Where is the heater grid located?
Wont hurt the engine any if you hop in a start cranking before the light goes out. You will however get a little more smoke when it first lights off.
I always plug mine in if the temp goes below 50f. I figure that these grids draw plenty of amps and that can't be good for the batteries. Also, I have a 53 block in my truck. Extreme temp changes seem to be the #1 cause for block failure. So I figure it is a little bit of insurance to plug it in.