There are many advantages of defining Rigid Diaphragm commands in the model. You can find out Center of Rigidity of the floor by using PRINT DIA CR command. You can also find out the actual story stiffness using PRINT STORY STIFFNESS command. Soft story checking gets the effect the stiffness of the rigid floor diaphragm.
However, you can use the above mentioned process to get the actual CM of a floor.