Managers should not have to depend on Project Creators to "share" correctly. If they are left off of the project sharing, they do not have visibility to their direct reports task assignments (unless you do wonky workarounds like creating a report run with the access level of system administrator).
Currently in Setup > Access Levels > you can set up rules for projects to automatically grant view access to the project creator's manager.
Likewise for Task Access Levels, you should be able to set up a rule to automatically grant view access to the task assignee's manager.