I do let it warm up in the winter 10 min or so.

but I have heard that it is bad on the tranny. The Tranny pump is running but no fluid is being circulated, so that makes the tranny pump heat up. so if you are going to let the truck warm up for something like 20 min in the mourning you should put it in neutral and set the parking brake. (just something I heard)

 

Gas cost too much to burn sitting in the drive way. I usually let the idel come down after starting, before I take off.