I usually being writing test cases and scripts at the requirements phase. When the client submits their requirements as to what the web site should include, I write the test cases to enforce this. However development wants to handle this is acceptable as long as it doesn't break, stress out the system, or violate the requirements document.
I also tend to tweak or add to test cases in the latter stages rather than add new ones. But this doesn't mean that I'll never do it. If a client changes the spec or requirements, it would require me to change the tests. In that case I might add or remove test cases.