When I first started remodeling I was taught the "DIY" way of installing crown (cutting 45s on the ends and trying to meet them up in the corner).  After much frustration and having to use half a tube of caulk to make the corners presentable, I took it upon myself to learn how to cope in crown properly.  However, we kept using these crown stops and the joints never lined up cleanly, even being coped.  One day I was sharing the saw with someone cutting shelf boards and so I just opted to do the old school double bevel cuts.  OMG!  I figured out the problem.  It was the stops.  Everything has lined up perfectly since I stopped using them.  No matter how much I adjusted or worked with them, they just do not allow for a proper angle to be cut.  It make take a little more time to do it old school, but my joints are tight now.