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".