Final answer:
True, a cost-plus system requires maintaining detailed records of costs to justify the final billable amount, as it is based on actual costs plus a markup for profit.
Step-by-step explanation:
A cost-plus system indeed requires that an agency keep detailed records of the costs it incurs while working on a client's account. In a cost-plus system, an agency calculates the costs associated with providing a service or product to their client and adds a markup or a percentage over that cost to determine the price charged to the client. This markup typically represents the agency's profit. Hence, maintaining detailed records is essential not only for transparency but also to justify the final billable amount to the client. As the agency must be able to articulate what the costs were and how the final price was determined, accurate and comprehensive record-keeping is critical in a cost-plus system.