My CRD Liberty I leave plugged in over night, and basically whenever I know Im gonna need it, every day LOL Why not? easier on the starter, battery, fuel system, and everything likes to work better a little wormed up. I know I do.
My 91 Cummins started at -20 a few years ago as I didn't know the block heater cord had an open in it. I fixed that and it started a lot easier from there on.
