I have it turned OFF. I think that is the right setting?
I think I'm getting closer to the problem. I have ruled out it being to do with ordinance. I tried a few missions, dropping bombs/rockets on left side only then restarting mission (clicking "Fly Again") and everything is fine.
However, I just tried a similar thing with fuel. Started the last mission "Ground Attack". Took off, switched the right centre fuel pump to OFF. Sped up time and flew around for a while until burned about 1/4 tank of the centre fuel tank. Had to put left aileron trim in to stop roll. Crashed the plan, clicked "Quit" then "Fly Again". Confirmed there was no aileron trim on indicator and both centre tank pumps were on. This time after takeoff, a/c had a definite roll to the left which I had to correct with aileron trim. At this point, all the ordinance and fuel should be reset at mission start so this was unexpected.
I think the amount of time spent in the mission and the fuel burnt affects how bad the uncommanded roll is on mission restart. The more fuel burn, the worse the roll. When i flew the mission, i was putting "LEFT" aileron trim to counteract right roll (right tank being heavier than the left as right centre pump was off). however, on mission restart, I had to put "RIGHT" aileron trim in to stop LEFT roll. This seems counterintuitive to it being an issue with fuel not resetting, so maybe it is still picking up the aileron trim from before the reset? Will keep investigating...