I guess we are talking about multirotors? What do you mean by “level with the direction of flight”?
Multirotors can fly in any direction by changing the attitude of the vehicle.
Tilting the motors in one direction would limit the ability to fly in other directions and would cause the copter to hover “nose high”. Some VTOL or STOL RC concepts do that using a H frame configuration.
With an X frame it is impossible to tilt the motors in one direction, because the arms are rotated 45° from forward/reverse or lateral directions.
If you want to get better yaw authority, you can tilt the motors on a X or H frame to add a thrust component to the torque yaw control.
Yea on multi rotors if the arms are not completely level due to the natural frame dihedral, you will always get different angles on the arm motor alignment compared to the direction of flight.
Our multi goes on long hour long missions at speed in one direction. So for most multis I understand it may be best to keep them even. However my question relate to our specific use, should I align them all with direction of flight.