And as for using speed brakes (not just for overspeed, but also undesired speed increases inside the operating limits), Boeing writes:
If the airplane experiences a sudden increase in airspeed, consider using smooth extension of the speed brakes to increase drag and to avoid large thrust reductions.
I don't, in anyway, share the opinion of Gretchenfrage. There is no problems using speed brakes during cruise what so ever (of course while keeping the hand on the level and the eyes on the airspeed indicator).