Just to list a couple of things that affect changing trim as you land,
Speed: As you slow down, the planes trim shifts forward, making the plane nose heavy.
Power: The prop on a Merlin powered plane such as the Mustang or Spitfire rotates to the right, which makes the plane rotate slightly to the left. As you throttle down the amount of rotation to the left reduces, so if the plane is trimmed to fly straight at full throttle it will start rotating right as the power is reduced.
Flaps: The flaps increase the amount of lift generated by the main wings, pushing the nose up.
Undercarriage: When the undercarriage is lowered the drag generated in its position pulls the nose down. The effects from the flaps and undercarriage will largely but not completely cancel each other out.
As you can see, setting up the planes trim before starting your landing approach isn't much good
