I rather mucked up their nice ganntt chart by insisting that content entry was bought way forward, before the site design was signed off. "Lets the get the content types defined and get some real content in there - give me any kind of cobbled together basic back-end you can" then we can see how the designs work with real content.
It actually worked very well - and avoided the death march at the end of the process where you have to port all the old content into the new system at the last moment.