For even more coolness, there's a doodad which allows you to select all of a given shape on a layer, so that you could change all your rectangles to rounded rectangles in one stroke, for example. I won't try to describe it here since I don't have a Mac handy but would encourage you to watch the video tutorial put together by Don McAllister at
http://www.omnigroup.com/products/omnigraffle/videos/ — if my experience is any indicator, you'll pick up something new each time you watch it while you are getting started.