I normally leave combat trim on, but I keep elevator, aileron, & rudder trim on my x45. If I start to compress in a dive, or I need to bring my nose around fast to put my guns on somebody in a turn fight, I hit the elevator trim. Costs alot of speed, but you can turn startling fast. Aileron trim I rarely use, only when I'm damaged, or cruising with an uneven dt/bomb load. Rudder trim I found pretty helpful in bombers. It gives you a much slower & gentler adjustment when lining up bomb runs, and doesn't really affect your speed any (thus, doesn't ruin calibration). Rudder trim is also pretty useful when moving into tight formations with squaddies.
I keep the combat trim on/off button on my pinky switch, so I can quickly cut it back on when I'm done with my adjustments and I want the joystick steering from a nice centered position. Practical example: Diving hard in a G10, compressing, and trimming out of it with elevator. As soon as it comes level I snap back on combat trim & can let go of the joystick if I felt like it.