Lots of information on this, even on this site. The proper way to warm up a diesel is to start it and drive away. Diesels are so efficient that they will probably never reach proper operating temperature at an idle, they will smoke, waste fuel and probably the result is glazing the cylinders. The jerk-ignition engines most of us have are most efficient at a certain rpm, probably very close to the torque peak where they are running loaded and burning fuel most efficiently. To see this effect most dramatically, watch the Sunday startup at Roche Harbour after the Grand Banks Rendezvous when up to 120 GBs all light up at the same time. The sheen in the water and the smoke is literally breathtaking. Once they all move off and start to warm up, the smoke vanishes.
Nobody is suggesting that you run your engine hard while cold, we are saying that warming up with a load is best. If you must idle for long periods of time and you wish to get your temperatures, put it in gear.
Then again, do what you want or do, what your buddies on the dock do or do what your grandfather did. This is perilously close to anchor-type and oil type discussions for voodoo and nonsense.
Don't believe everything that you think.