When developing a web app, in my case a ASP.NET MVC app, but this question isn't platform specific, when you do you add the infrastructure for membership, roles and authentication?
I've actually done it both ways.... 1. Start developing the app with membership/roles/authentication being one of the first milestones and 2. Wait until most of the major functionality and requirements are implemented then add in the membership/roles/authentication infrastructure.
Is there a best practices for this or it is personal preference?