Lets try using the example of a common scenario.
Imagine an aircraft has just lifted off the runway following a normal takeoff. The pilot flying notes that a light constant right roll input pressure is required to prevent the aircraft from rolling to the left. Normally not a condition of great urgency, but no pilot is satisfied with the feel of an out of trim airplane. Bad form because it requires more effort in order to precisely and smoothly control the flight attitude. Very early on, the pilot will check the position of the inclinometer ball. In many cases, only verifying something allready suspected and reacted to with rudder pressure. Suppose the ball is displaced to the right, as it might be in this case. Right rudder pressure would be applied to center the ball. Right rudder trim is then gradually applied and rudder pressure reduced simultaniously until rudder pressure is no longer required to center the ball. If done smoothly, nobody will feel it being done.The need to hold right roll input would likely be reduced or eliminated. Now, with a centered ball, aileron trim may be used to relieve any remaining need to hold roll pressure on the controls. This is all done in a matter of a few seconds by experienced pilots out of long established habit. The force imbalance condition is now corrected enough to fly comfortably. Every day stuff.
Why should a "wing heavy" condition be corrected in this manner and not the other way 'round? To prevent "cross trimming", a condition where the airplane is flying with a yaw tendency acting in one direction where the resultant roll tendency in the same direction is countered by trim generated roll input in the opposite direction. The wings could be trimmed nearly level using this method but the airplane would then be flying with it's longitudinal axis not alligned with the flightpath. The additional drag penalty from the sideslip angle plus the constantly displaced flight controls is not desirable. In at least one type though, this condition seems to dampen dutch roll tendency in place of an inoperative yaw damper!
If aileron trim was stuck in a position that induced a rolling tendancy, rudder trim could be used to hold rudder input and counter the rolling tendency. The aircraft would then be in a "cross trimmed" condition, but would tend to maintain a constant slight bank and a heading slightly offset from the flightpath as above. It could be controlled and landed safely with inoperative aileron trim.
Through heavy use and multiple maintenance adjustments, some jets "of age" that I have flown just don't fly straight! Rudder trim right for climb and left for descent just like a prop single. We like to say they are "bent". Probably just poorly rigged.
Best regards,
Westhawk