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Graduated Cylindrical Shell (GCS) model is a widely used tool to determine direction, kinematic and orientation properties of Coronal Mass Ejections (CME) using multi-viewpoint observations from SOHO and STEREO A&B coronagraphs. In this study, we estimate the subjective uncertainties typically seen while deriving these CME properties by comparing the GCS model results reported in multiple studies and catalogs for 56 CMEs. We find that the GCS estimates of latitude, longitude, and tilt show an average uncertainty of 5.7, 11.2, and 24.7 degrees with standard deviation of 5.5, 12.7, and 19.7 degrees respectively. We found that the uncertainties in estimated latitudes are correlated with uncertainties in estimated longitude, tilt, and speed, showing that some CMEs are inherently difficult to fit than others. We then introduced these uncertainty values in our 3-D magnetohydrodynamic flux rope based modified spheromak CME model to figure out their consequences for space weather prediction. We find that much better CME observations are required to reliably predict magnetic field of CMEs at 1 AU using flux rope based models, since the uncertainties in estimated GCS values can result in large differences in 1 AU signatures, especially for CMEs launched away from the Sun-Earth line.