In my clients I have experienced two approaches in how a project manager is assigned:
1. When there is a formal project selection process is used a PM is assigned to guide the project through the selection process – e,g, prepare high level estimates, cost/benefits analysis, prepare capital appropriation requests, etc, However, this PM may not be the same PM that is responsible for the project’s delivery should it make it into the portfolio. I have seen this approach in larger clients who select the majority of high visibility or high cost projects as part of the annual budgeting process which typically is earlier than project startup. Typically PMs used for this purpose have skills particular to the selection process, internal budgetary processes and are adapt in the political landscape of the organization. The delivery project manager is assigned closer to the project startup.
2. In smaller organizations regardless if there is a formal project selection process the PM is assigned at the start of the project selection process (or startup if there is no formal selection process). The PM is accountable from the ‘cradle to the grave’.
Clearly there are pluses and minuses to both approaches. I suspect there is no correct approach and is ultimately based on the organization’s culture.