Perhaps the real benefit of applying a moss killer prior to raking is that the moss killer will desiccate the moss to some degree thereby reducing its bulk. So, if you have a bad enough problem applying a moss killer before and after raking may be very beneficial. Allow several weeks between treatments with the applications being made in cool wet conditions otherwise you will start to blacken the grass. In addition ensure your moss killer is low on fertiliser (nitrogen) content as a double application may cause growth or disease problems. Straight iron sulphate based moss killer would be the preferred choice in this instance.