As a rule of thumb, designers retain the original format files such as the actual photoshop, illustrator or indesign files. The client gets the resulting jpg, eps, PDF, etc. This protects your work and your future as a designer.
I've seen it happen where designers turn over their original format files, and the client then turns around and hires an inferior designer (like thru outsourcing) for pennies on the dollar for future projects.
I believe the more important issue here is to make the communication for the end goals and requirements a top priority during a preliminary meeting with the client. If they want the original format files, it needs to be in the contract. And then charge a lot more for including them. Also reserve your copyright to your designs, and if the client wants the copyright, charge more for that too.
For a comparison, consider a custom motorcycle builder. A client wants a really awesome bike. The builder prototypes, designs, and assembles it. At the end, the client gets the bike. But by providing the client with the original format file, its similar to giving the client all of your tools you used to build the bike in addition to the bike itself. Now the client doesn't need your services anymore.
If the client insists they need the original format for X, Y or Z reasons, almost all the time a flattened format will work just as fine. If they still insist on the original file, then insist it's a standard contract that the original format file comes with a much higher price.
And if you need contract examples, check out http://docracy.com or reference the AIGA standard form of agreement at http://www.aiga.org/standard-agreement/