when checking AT fluid level: level ground front-rear or left-right
 

When checking transmission fluid level,

I should place my car on level ground.

Between front-rear levelness and left-right levelness, which is more important?

There is no more important. There is only one important, and that is level. Think of the oil pan on your transmission as a baking dish half full of fluid. Put a depth gauge in one corner, and raise a back or opposite corner. Your gauge will say you have more fluid than you do so you are running your trans low on fluid. Raise the end with the gauge and it will measure less than is avtually in the dish so you add fluid and overfill causing foaming in the transmission. Only when the car is on level ground while still not becoming anal about it do you get an accurate reading.

Hope that helps with the question,
Peter