Why would you overspeed? The barberpole will move up at the same rate as the increase in IAS when flying a MN. To end up with a selected MN into VMO takes a while. The only thing that could lead to overspeed is a change in atmospheric conditions. As you start your descent the a/c will increase the rate to maintain the increasing MN so you can end up with a high rate for only a 2000 ft descent and that could potentially cause an overspeed or a TCAS. Thats why I prefer VS instead of open des/lvl chg for small altitude changes.