Yes, but be wary of showing a prototype UI which works as if the functionality/back-end is also working. Even if you stress that its not, the client will understand that the functionality is already working if they can see it, and won't understand what you're doing afterwards or why the back-end is taking so long. Be especially careful with making assumptions about complicated functionality and back-ends, as they are difficult to estimate and clients grow very frustrated after they've already seen everything "working".
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!
