I don't believe it should be handled only one way or the other as renderings serve different purposes, project scopes are different, clients are different, etc. At our firm we need to do ALL OF THE ABOVE. We're a very small studio by comparison, 45 employees over 3 studios.
Bill as an Add. Service: This type of service is typically geared toward producing marketing renderings for sales, or anything that serves our clients own needs. This is the method we prefer as the scope and fees are clearly outlined in a separate contract.
Billed into Arch. Fees: We'll do this when producing renderings for the sole purpose of communicating design intent, or anything that serves our needs for communication (public hearings, presentations, etc).
Outsourcing: As stated above, we're a very small studio by comparison to the others here. Thus we need to be very efficient with our time to cover the work load. We'll typically outsource when timing is critical and spread the work over several private studios in Virginia, Toronto, and California. The billings are then handled by either situation listed above.