Say you're flying at CI100 and the airplane is flying at .80 and you just want to slow it down a bit. Selected mach works really well. Maybe choose .79 or .78.
Selected speed works well if it's very turbulent and even selecting the middle of Vls and Vmo you get nervous how close it's getting to either end (usually a heavy plane close to max FL). The a/thr will be much more responsive in selected speed.
Be careful though with selected speed, the thrust will come right off if you get too fast. On the other side of a wave when you start loosing airspeed the thrust will take ages to roll back up from idle. A few times in really bad mountain waves I have just turned the auto thrust off and that was much easier, while descending into thicker air to lower Vls and have a bigger window, even if the turbulence is worse down there.