No, you're right, but the key word in your question is "usually". By convention, when a user creates a domain, she will specify that its web directory as /home//. But she's free to use any other path in her home directory for a domain even if it's unrelated. For example, the web directory for "foo.example.org" could be /home//bar/baz/qux.
I'm curious to know if there is a way for a shell script look up the web directory for a domain. Using the example above, if the user were to run the script and give "foo.example.org" as an argument, could the script somehow determine the web directory is /home//bar/baz/qux?