I have been making my seed balls flat, as the area I want to seed is a very steep slope next to my driveway. I don't want all the balls to roll to the bottom of the slope!
Warren Brush from the Quail Spring Permaculture site in Santa Barbara, CA has taught me that he makes seed triangles(or pyramids as I would describe them) so they don't run down sloping hills he covers.
Thus far, we've had great success at Hayes Valley Farm (San Francisco, CA), mixing up a huge batch of clay, compost, and seeds at the beginning of our volunteer work days. Whenever people get tired sheetmulching or any other activity they go to the gathering / classroom area and relax with others making seedballs. It's a great activity, low energy, and creates a lot of connection for the volunteers to the earth and one another.
Peas and borage!