Good question, much of the guidance found at FAR 1.602-2(d), DFARS 201.602-2(d), and DFARS PGI 201.602-2(d) is silent on exactly when/where in the process a COR should be formally designated.
That said, it is highly prefered that a COR be designated prior to contract award, see DoDI 5000.72 Enclosure 3, Paragraph 1.c. (page 9) and FAR 7.104(e).
In actuallity, the earlier a COR(s) can be asigned the better!! If they participate in pre-award activities (e.g. market research, defining the requirement, acquisition planning, selecting the contractor, etc) the COR, the requiring activity, the KO (contracting officer), and the contractor have an improved probability of contract success.