It's tempting to create a role for each job title that is going to access the site. Don't do it! "Role" might sound like a synonym for "job," but don't be fooled -- it's just a friendly term for a collection of privileges. Roles are convenient in small quantities, but they get to be a hassle when you get more than a handful. Here's a tip: if there are too many columns to fit on the "Permissions" screen without scrolling, you probably have too many roles.
Choose your roles carefully, and try to limit their number.