Appreciate the advice, a quick look through a StackExchange (generally good forum) has plenty of views both ways. Cobbling them together, it appears as though:
1) Leaving in D creates extra drag (as previously stated in this thread). This means the engine would idle low. To compensate, ECU modulates the IAC which means more air, more fuel, more power/energy. This excess energy culminates as heat in the ATF. Too much of that would indeed be bad.
2) Leaving in N would therefore result in less drag = slight fuel savings at idle at the expense of wearing out shifter bushings etc. Apparently, it also allows one of the clutches to slip slightly, theoretically resulting in more wear. I guess this is why people say NOT to rev in N/P. This is especially pertinent in the ZF autos which apparently don't leave the ATF pump running when in N/P. The ATF pump on the AW7x does continue to run in N/P, thus it's not as much of a problem (although maybe don't make it a regular thing).
3) Something about the AW55 (basically a non-overdrive AW7x, right?) automatically selecting and deselecting N when at a standstill AND in D AND with foot on the brake... Will have to look out for that on my commute tomorrow. Allegedly this kicks in after 2 seconds, and kicks out again when the brakes are released?
Here's the link to that thread if anyone is interested:
https://mechanics.stackexchange.com/...c-transmission