As mentioned earlier, the mass in the model should include everything except the actual motor, the mass of which Rocksim will account for when you load the engine(s) into the model. Remember the 'chute, cordage, protectors, etc.
I almost never try to get all the masses right anymore and often don't even add centering rings. In the last step, I measure CG and mass and then iterate to find the required nose weight, if required.