Agile software development principles state that the needs of your project should drive the artifacts developed. Need to explain something complicated to a group of people who will implement it; then create the minimal set of diagrams to help convey the idea. Then talk to them and see what questions they have. If as an architect you want to create a class diagram because you are worried that the developer will not select the ideal class names or create your nifty inheritance hierarchy; then you are most likely wasting your time.
I believe that a majority of UML diagrams are create after the fact, just so engineers can try to convince people that they know what they are doing.