You're essentially powering off a virtual hard drive. Upon shut down, Windows has to correctly off-load it's RAM contents back to the virtual hard drive so that next time you boot up things like registry changes are up-to-date.
Improperly shutting down a VM has the same software/OS implications as pulling the power-cord on your physical machine aside from the risk of electrical damage to components.
I recommend to always Shut Down the VM through the Start Menu, send the shutdown signal, or simply save the machine state.
The only reason you would simulate a forced shutdown is any reason you would force a physical machine to shut down such as overloading it and causing it to freeze.