But the reality is, you do both. First of all get the plane skid/slip free (rudder trim) and then get the ailerons trimmed. If the plane is really bent, you have to re-do it every time you change speed (and in a turbo-prop, every time you change power as well). Unfortunately, it appears very difficult to get a straight aircraft. But considering how big they are, it is amazing that they are built to the tolerances that they actually are.