I think this is going to be completely personal opinion. If it is less than 40F outside, I will go out 15-20 minutes before i need to leave in the mornings and let my truck idle, to warm up the block but also to warm up the cab so I'm not freezing while driving. When the summer rolls around and its over 110F and I will not be out of the truck for more than 20 minutes, I let it idle and keep the air running. Now does this waste fuel? To some I'm sure it does but it does not to me. The old diesel motors, before all this EPA crap was put on, could idle all day and all night and it wouldn't hurt the motor at all. Gas motors it isn't great to idle it, but also I have never heard of anything major happen that was solely based on idling a gas motor. But again I think its all based on personal preference. I know people that will idle their truck while they are in the grocery store for 2 hours, I also know people that turn their truck off even if their just checking their mailbox for 2 minutes.