If you leave the engine on,it should be ok to coast in N.
D to N,I don't see anything that is stressed.
N to D well does it engage smooth when you go back to D?
If so,it should be ok as well. If for any reason there is a clunk,jerk whine whatever does not feel right when going back to D, in that case I wouldn't be doing it too much.
Because there are so many things in an A/T that can wear at different rates and effect shift timing or engagement of components ( servos,pistons,springs,clutches,seals etc.) that's why I would go by feel and your own judgement instead of a rule of somekind.
On the plus side if you do blow up the trans,it will be a good excuse to get something with a proper manual tranny
