In his most recent post, Martin Fowler talks about the necessity to teach clients about the value that resides in the tests his company delivers with any software solution they ship (and how, without this recognition, tests will be altered and mangled by the client, thus will lose most of their intrinsic value).
Indeed, it is interesting to recognize that clients might consider that the only valuable outcomes of a software project are the software itself and its documentation, and consequently ignore other valuable resources.
In that matter, I think there is another valuable outcome that exists but is generally ignored by clients: the source control revision history. Understanding how a software has been built not only reveals a lot about the professionalism of the builder but also helps understanding how the application took form and reached its current incarnation, which can further help making it evolve gracefully.
As a software consulting client, the most convenient option is probably to open a dedicated source control repository for the consultants and grant them access both internally and externally (they will very often work off site). After the project has been completed, the source code and its history can then be moved from this repository to the client's private one. Similarly, an alternative option is to let the consulting firm use their own repository provided a export/import path exists between the consulting and the client source control tools.
To sum it up: cherish your code and its revisions! After all, this should come as no surprise since everybody knows there is a lot to learn from history: this is just a different context!