I've installed quite a lot of crown molding in my time as a finish carpenter and remodeling contractor. And I can tell you that there are times when it is nice (and sometimes necessary) to use crown blocks. Visually they add quite a fine touch to your crown molding. In the case of cathedral ceilings that meet horizontal ceiling they are virtually a must have. Crown molding applied to a angled ceiling will not match a horizontal crowned ceiling. Try as you may the geometry does not add up. The profile of the molding (on the angles ceiling) will not match the molding profile of the horizontal (trust me on this). And so this is a great place to apply corner blocks. You can read more here: Corner Molding Blocks