I have a 03' 6.0L and if it's pretty cold out it takes a few minutes for it to start running smoothly, diesels aren't real big fans of starting in the cold!

Mine has always started fine in the cold even without the block heater plugged in. It might take a few tries to get it fired, but once it's fired it always idled pretty smooth. The block heater does help tremendously though, and I use it all the time now to aid in quicker warm-ups, quicker warm-ups mean my veggie system kicks over faster.