Anthony,
The FMC Database will make sure the correct TA is displayed. Thats if the airport is in the database. If it is not then you can simply put it in manually yourself.
In the real world the TA is actually not particulary important. To avoid level busts you would set 1013 whenever you are cleared to a FL and QNH whenever you are cleared to an altitude, the TA doesnt really come in to it. ( See current tech log thread)