LONG periods, it is VERY slightly better to put it in Neutral... just don't forget about it and try to drive off in 'N' when the lights change... otherwise the revs will rocket, and you'll go nowhere! (also, it won't engage 'D' until the revs reduce, since it automatically avoids 'clutch-dump' starts).
However, the reasons for doing so are minimal. -Basically it keeps the 1/3/5 clutch pack cooler not to be slipping very lightly when stationary.
Also, when the engine is cold and you're asking for heat, it tries to be 'clever' even when it's NOT in auto... so it keeps the fan speed down until there's actually some warm air to USE... then it speeds up the fan.
Once the engine is up to temperature, (and assuming you're not changing the temperature setting up and down more often than a whore's drawers) the fan speed should be steady.
It can't be in auto... as soon as you adjust the fan speed, it comes OUT of auto.
Keith