@Eric.

It doesn't really matter. Standard 180 degree servos do not have a left and a right so to speak. They start at 90 degrees (Center) then you either lower the values (1 is the lowest on ARC) or set the maximum value (180 is the highest). So the min and max movements will decide left and right, or up and down movements depending on the way you mount them.

EDIT:

Ill give you an example. A servo moving a left arm and a servo for the right arm.

If both servos are resting at position 90 degrees, the arms would both point forwards. If you move the right arm to point up, that servo would move to 180 degrees. If you move the left arm to point up as well, that servo would need to go to position 1 degree. If the left arm servo moved to 180 degrees as well, it would be pointing down.

I hope that helped to explain things. :)

@bernard eric.

Just to add to this. If you use DC motors instead of servos, you could potentially have left and right motors. All that would need to be done is to change the motor wiring on the motor controller, or change the PWM values.