Guys, I'm wondering how and when to use aileron and rudder trims on fs9 pmdg legacy boeing 737-800? what are they for? I'm trying to be a certified pmdg legacy 737ng flightsimmer here.  Help would appreciate it.  

You'd use aileron and rudder trim to maintain track when hand-flying in a crosswind, either on take-off or final approach.

If you are flying by hand on finals then I would have thought that maintaining track by trimming is not a good idea. Use of the rudder pedals to stay on track means that just before touchdown you can point the nose in line with the runway by applying the opposite pedal; difficult to fiddle around adjusting trim at that point, and applying opposite pedal against the trim......... I admit I have never flown a 737, only single engine props. Never had an aileron trim either, only rudder trim, which was used to fly in a straight line from point A to point B without use of rudder pedals and without tiring a leg.

Don't mess with aileron trim at all, especially with an A/P engaged. Sometimes in the real jets, you need a little up at cruise to fine tune the jet to fly as clean as possible, but generally leave it at zero...

The only time you really need rudder trim is with an engine failure to relieve foot pedal pressure. If power is changed, then the rudder trim requirement is changed as well. Generally fly with your feet first, and then trim it out. More thrust, more trim, less thrust, less trim as power changes for airspeed requirements. Foot or rudder pedal pressure first, then trim it out with rudder trim.

Aileron and rudder are required for crosswind takeoff and landings, but not aileron and rudder trim. In a heavy jet, aileron needs to be used sparingly as excess aileron can cause spoilers to come up and induce drag on takeoff. I agree with Rogwen on that one...