With the Phantom 2 Vision+ you can already achieve a very good relative accuracy. The GSD that you typically obtain flying at 50 meters is between 1.7 to 2.7 cm/px.
However, if you are interested in absolute accuracy especially for the accurate professional topography we would definitely recommend to use GCPs. Especially knowing that the calibration of the altimeter of the Phantom 2 Vision+ is more or less 10 meters. This means that without GCPs your model could be shifted more or less 10 meters from its real position.
For more information about GCPs:
For more information about relative and absolute accuracy in Pix4Dmapper: