With the trim settings.
I find that if I don't set any trim other than elevator, that when flying that I have to put quite a lot of left aileron otherwise she rolls to the right all the time.
Is this right?
Assuming you don't have any controller issues and are calibrated correctly in FSX, the main thing to remember about trim settings is that what you read in the manuals are "suggested" settings. Each aircraft even of the same type will be "trim specific" as to its exact configuration at any moment in time. You can visualize this by considering that if ANYTHING CHANGES in flight, and this includes fuel and expendables burn and especially airspeed, required trim will change as well and have to be reset.
When flying a fighter like a Mustang, you should be constantly having to attend to trim both aileron (as fuel is burned from one wing tank or the other) and elevator (as airspeed changes so does angle of attack and thus trim changes as well. ) Note also that in the lower speed range where angle of attack is higher, a Mustang will be especially trim sensitive due to power setting.
All in all, the bottom line is that the trim wheels in a Mustang are always in play as you fly the airplane.
All this having been said, it your problem is severe it's most likely a controller problem. I've checked the flight model pretty carefully and it's extremely good as compared to actual behavior.