Opamp Vos trimming in a single supply circuit
You can get some idea of the offset voltage change vs. supply voltage from the below figure taken from the datasheet: 



Adding the negative 12.6V is equivalent to changing the common mode voltage by -12.6V. The change looks like it could be some tens of uV, but probably typically &lt; 100uV. Something like -0.3V might be better than -12.6. 

Usually it's better to trim with an output voltage enough offset from 0V that the amplifier is working well, yet fairly close to ground so as to minimize iterations if you are doing span/zero calibration.