This has been discussed a few times before. The main thing, as said above, is make sure that clients have details on how to access the domain and the server / site files, so that the site can be handed on to another developer in the event of your demise.
We've also seen examples of personal sites with great content that have eventually disappeared after the site owner died. It's rather sad. Ideally, we'd appoint a colleague or someone else as an executor of the site so that it could be maintained or offered to someone else if we disappear.