I actually use automatic route generation for my api Rest, using the FOSRESTBundle, also I use NelmioApiDocBundle to generate the api doc.
To generate routes for the api this I have in my routing.yml
users:
type: rest
resource: Project\RESTBundle\Controller\UsersController
But for some actions I want to set my custom routing... If I try to add another route rule for an action it simply ignore it and generate the automatic route.