Had to do that, once, to maintain minimum speed for that altitude, in a mountain wave at FL430 crossing the Rockies SW of KDEN.
Could the a/c have flown slower without stalling? Yes. But minimum speeds are there for a reason.
In a 'block altitude', we descended 700' at max cont. thrust at the minimum airspeed, bottomed out, and climbed right back up to FL 430.