Automatic newcomer needs advice

Hello! Since yesterday I can call my first automatic car my own I have a question. More precisely I have a Qtronic transmission. If I now stand at a red traffic light, you switch to N (just like at the switch to spare the rear position) or can you stay on D? If I stay on D it easily vibrates in the car, you notice that it wants to drive away but is braked. What is more harmful now, the “braking” of the transmission or the additional switching organ g from D to N and back again? And still a little bit hot, how long does the gearbox take to become “warm”? I drive about 40km of easy route daily, 35 of which are motorways and the first and last 5km 100km/h speed limit (city highway). Since I try to speed up to 100 as slowly as possible and only after that I drive 130-140km/h. Right so? mfg, emyr