You know what would be helpful? Some sort of graphic (or digital) indication of the amount of trim on each axis. MSCFS3 shows this (digitally) when you press 'Z'. For ex. I know that for level flight in a B25C I need -48 elevator trim. For climb I need -36. Its easy to get back to these points rather than going by 'feel' all of the time.
Back on topic. I used to map to the rotary knob, but on reading some advice in the Pacific Fighters forum I now have the elevator trim mapped to the second hat on the X45 stick.