Successive Interference Cancellation (SIC) is a key element in a Non-Orthogonal Multiple-Access (NOMA) receiving terminal and must be properly tuned in order to achieve the optimal performance. To this purpose, receivers have to be informed about which signal components have to be decoded before decoding their own. In this framework, most literature refers to a proposition reported in [1, Summary 6.1] for broadcast Gaussian channels implying that the optimal decoding order consists of decoding the weaker user signals first and then their own. In this work, the blurred contours of the proposition are brought into focus by considering the widespread Proportional Fairness (PF) criterion to harmonize the user rates. Theorem 1 is the main contribution assessing the optimal SIC order of a NOMA channel under a PF-optimal criterion. It is observed in the paper that the optimum SIC order may change with different utility functions which are concave and monotonically increasing with the users' individual SINRs. Numerical results are also presented to confirm the validity in a fading environment.