The training rate still goes down when enough skill points are accumulated to reach next star, from my observations. For a fast trainer from 4*-5* is between 20-25% drop (not typed up the rest of the data yet). You only see the drop in actual sp gain over a longer period because the gains are so variable but it occurs whether assigned or not.
Market Value is ofc related to training rate but you need to remember it is MARKET value and the market does not recognise unassigned points.