Err, I was going to post but I realized I don't know that much about it. I think it depends on the airfoil, dihedral, wing position, and if you want to fly inverted or not. On a glider, a huge amount of differential is used (like 4:1). I believe the reason is that your flying at a very high coefficient of lift and the down aileron causes a great amount of drag while the up aileron decreases drag. On an aerobatic airplane, your flying at a very low coefficient of lift and the up and down aileron have close to equal drag. If your inverted, the angle of attack is reversed compared to upright and the differential (due to aileron drag) is also reversed. However, the other effects (dihedral and wing position) are not.

OK, experts can take over and correct from here.