idling for prolonged periods of time causes cylinder wash and won't lubricate your motor well and can cause damage.
a lot of people rig a pto throttle on that raises your motors rpm and prevents this.
I usually use my block heater, but won't idle my truck longer than 30 minutes... some people idle them forever
98 12v Cummins Twin Turbo Diesel. Bosch P7100, NV4500, D80h...