Component Balancing - The compressor wheel, and turbine wheels can be balanced individually, before being installed in either a rotating assembly, or into a complete cartridge for for a cartridge balance. Just balancing the individual components and slapping them into a cartridge or onto a turbine wheel, is NOT a good idea. The components need to be assembled and balanced dynamically as either a rotating assembly, or in a assembled cartridge.
Rotating Assembly - Generally consists of a assembled Turbine Wheel/Shaft, along with a Compressor Wheel, Locknut, Thrust Collar and Thrust Sleeve.
After the parts are stacked on the Turbine Wheel, the Locknut is torqued down to factory spec, and alignment marks are scribed on the nose of the Compressor Wheel, the Turbine Wheel/Shaft and the Locknut. This is to ensure that after the assembly is balanced and taken back apart for installation into the bearing housing, that all three of these components remain in "phase" with each other when reassembled.
The assembly is placed in a specialized balance machine, and additional material is removed from the bottom of the Compressor Wheel and Turbine Wheel with a hand or bench grinder, until the assembly is balanced to spec.
Once balanced, the components are disassembled, cleaned, and installed into a assembled bearing housing to form a cartridge, or CHRA. This is how most shops used to do things, before cartridge balancers were made available.
Cartridge Balance - The balanced Compressor and Turbine Wheel are installed into a assembled Bearing Housing, and the entire CHRA is placed into the balance machine.
A small belt, or a directed air jet spins the Turbine Wheel on the CHRA and material is removed from the Locknut, Compressor Wheel Nose (in some extreme cases) and from the small "nut" on the back of the Turbine Wheel. This is the most accurate, and cost effective way to balance a CHRA, as all the components are spinning dynamically, and are riding on the actual bearings in the Bearing Housing.
Most of the larger rebuild shops use this method, because it's quick, and most importantly, accurate and allows your turbocharger to enjoy a nice healthy, life.
When it's spinning as fast as some people push their turbos. the smallest imbalance or error made while assembling the turbo can dramatically shorten its operational life.
Assuming you don't cock it up by running it out of oil, dropping pressure while operating or letting bits of your engine contaminate the oil supply.