On Boeing jets, in cruise when one needs to reduce speed, the Speedbrake will be the first option.
Boeing does advice strongly against idling your thrust on cruise. The SPEEDBRAKE is there to accomplish the mission, but do it gently, like everything at altitude.
I seem to remember that AB recommends the same.
By the way...if in doubt: all automatics OFF, go to DIRECT LAW, fly the aircraft raw data like any other aircraft in the world .
First lesson I got during my TR on A320 now 14 years ago...is that still valid? Am I completely misled in this?