I very seldom use the built-in roles. I always build the roles custom. but, I only build them as needed. Not all roles are ever needed on all databases. And, some roles are unique to some databases. I'm a fan of consistency, but only where it's actually serving me. I don't like consistency just for consistency's sake.
"The credit belongs to the man who is actually in the arena, whose face is marred by dust and sweat and blood"
- Theodore Roosevelt
Author of:
SQL Server Execution Plans
SQL Server Query Performance Tuning