Numerous methodologies for subjective quality assessment exist in the field of image processing. In particular, the Absolute Category Rating with Hidden Reference (ACR-HR) and the Double Stimulus Impairment Scale (DSIS) are considered two of the most prominent methods for assessing the visual quality of 2D images and videos. Are these methods valid/accurate to evaluate the perceived quality of 3D graphics data? Is the presence of an explicit reference necessary , due to the lack of human prior knowledge on 3D graphics data compared to natural images/videos? To answer these questions, we compare these two subjective methods (ACR-HR and DSIS) on a dataset of high-quality colored 3D models, impaired with various distortions. These subjective experiments were conducted in a virtual reality (VR) environment. Our results show differences in the performance of the methods depending on the 3D contents and the types of distortions. We show that DSIS outperforms ACR-HR in term of accuracy and points out a stable performance. Results also yield interesting conclusions on the importance of a reference for judging the quality of 3D graphics. We finally provide recommendations regarding the influence of the number of observers on the accuracy.