I'm not afraid of a temper tantrum, it shows the client cares. Showing a basic UI that was generated by the use cases will confirm that you understand the client needs, reduces miscommunication and clearly highlights the feature creep you should be charging for. It helps the client sell the project to whoever she reports to. It's easier to change a UI and it's a stronger emotional lock-in for the client (so she doesn't cancel). Win all around!