As with many technical specifications that are used to compare cameras and lenses, much more is often made of the differences they exhibit in precise, controlled testing environments than the effects of those differences in real world situations.
In the case of the EF 50mm f/1.2L, the focus shift issue seems to come into play most often when extension tubes are used to achieve Minimum Focus Distance (MFD) considerably shorter than the 17.76" MFD the lens is capable of when mounted directly to the camera. If your subject is 3' away, that is twice the MFD of the EF 50mm f/1.2L and I would not expect the minimal amount of shift under those conditions to be detectable unless you were shooting very precise tests at high contrast test targets. With flowers as your subject, the slightest breeze could move them further during the time between focusing and exposing the image than the effect the focus shift would have on your photo.
It is also conceivable that if you are using Auto Focus your body/lens combo is slightly out of calibration in the opposite direction and the AFMA error and the focus shift are effectively canceling each other out. If you are focusing manually this would not be a factor.