Hi Thomas,
When checking the current user's roles you should always use WebUser.IsInRoles not siteUser.IsInRoles to avoid un-needed database hits. siteUser.IsInRoles is used when the user whose roles are being checked is not the current user, but it is a little more expensive becuase it does hit the db.
Note that when you rename a role, it does not really rename the role it only changes the display name, so you must continue to use the original name in code checks. This makes it possible for example for you to rename the Admins role to something more localized but without breaking the system. If you really want to rename a role you should delte it and re-create it. Built in admins role is of course not allowed to be deleted.
Hope it helps,
Joe