Yaw and aileron trim to center is part of takeoff trim, since you don't want to takeoff with an A/C that will rolls inverted as soon as you lift off. With asym loadouts you dial in lateral trim after pressing T/O trim.
The yaw dial will be mechanically centered when you press the T/O trim.
I think I read somewhere (might be on these forums) that the FCS reset should reset any trim in flight, but it's not implemented.