In general, it is safe to take the approach that whoever pays the bill for the service is the owner of the data associated with it. This "he who pays the bill owns the data" approach is simple and it makes the most sense. Consider that the individual paying the bill for the services is the individual who is financially obligated for what occurs with the service, so it makes sense that they would have authority over service access and usage.
It is quite common in outsourced and online accounting models for a professional firm to subscribe to services or solutions which help them support various processing needs for their clients. Solutions such as Bill.com or SurePayroll to provide tools to assist professionals in efficient delivery of various process-support services, such as bill payments and approvals, or payroll processing and reporting. These tools are utilized as part of the professional service offering, and are generally not directly exposed to the client users (other than in specific contexts, perhaps). Separating the client from these systems is usually not difficult; the professional simply stops using the solution for that ex-client. Since the transaction information from the solution ultimately integrates into the accounting data file, the accounting firm can simply return the accounting data file to the client without losing their process support data in the online service.
On the other hand, if the client was the subscriber to the solution and the accounting professional was "invited" to participate with them, the separation would mean that the accountant no longer had access to the online data, and the client would retain use of the solution.
In contrast to a process-supporting solution, separations become far more complicated when the online solution includes fundamental tools for the client like general business application access and data storage. Consider that a business decides to use SmartVault for its document management needs, and also wants to connect documents and files directly to transactions in their QuickBooks accounting system. In this situation, the accounting data and the document vault are closely connected, and contain a wide variety of valuable business documents and files.
When the solutions are both run as online services, where the QuickBooks applications are hosted along with the integration for SmartVault, both the accounting professional and their business client can work more closely and in real time, creating much more value in the relationship. If the relationship does not work out, however, separating applications and data can be a frustrating process for both parties if there isn't a clear understanding of who gets what. It would be easier perhaps if the question centered on an accounting data file, but in these situations the problem extends to questions of ownership of source documents, working papers, and even application software licenses.
Accounting professionals need to protect the value of the work they perform on behalf of the client, and the business owner needs to have their business information and applications. Clearly understanding how to orient subscription based services to protect the interests of both parties is an important element in providing the highest level of professional service to clients. In some cases it makes sense for the professional to own and control the subscription, particularly if the service is an element which supports professional services delivery.
These tools help you provide services to your clients, and the client benefits from the result of use of the solution. If the client leaves you, then it is up to them to "tool up" their own operations to handle those processes. In other cases, it makes far more sense for the client business to own and control their online services, and invite their outside professionals to participate.
The benefits of working together online are still there, and the remote access and mobility benefit the business owners and team members as much as their remote professionals. The accounting professionals can preserve their working papers and other work product on their own systems, drawing a clear line between their retained data versus that of the client and making a potential future separation much easier to facilitate.