[smuzz]

Yeh, you've got the fundamentals mate - differing amounts of up and down travel. Whether you need it on the ailerons is a function of the wing type, amount of dihedral and aerofoil. For example, if you're using one of the KFM aerofoils, airflow might be turbulent above the wing and laminar below - in that case you might want more throw up, than down.
It's sometimes also used when the plane demonstrates an adverse yaw (the ailerons are used to roll in one direction, but the plane yaws against the roll - this happened to me when I built my first trainer off plans and I ... made a few ill-considered alterations to the design).