To determine on which axis the additional controller should be placed, you can select a joint and use Translate manipulator axes as a reference.
For the convenience of controlling the character, the additional point should be facing forward. This way, you’ll be able to use this controller to adjust the rotation of the corresponding body part as well as its direction.
For certain characters, quadrupeds for example, it is convenient to direct the additional controller upward or downward.
On the limbs, place the additional controller orthogonal to the main ones (see Limbs to learn more about rigging limbs)
For the head, it is most convenient to place the additional controller on the surface of the character’s forehead, or on the tip of the nose.
When you rig shoulders, you should probably move the additional controller to the upper part of the shoulder so it would be better suited for adjusting the rotation of the shoulder. For the hands, set the controller in the direction of one of the finger joints.
And don’t forget that you can also place the additional controller anywhere you want by moving in with the manipulators.