Why would Boeing say climb or descend at a specific target speed, but in cruise, do whatever?
In cruise, either leave the A/T on and set 280/M.76 or turn it off and set the suggested N1 (which is the power setting for 280/M.76).
I'm all ears to differing opinions.
Next time you have "struggled" to FL410 have a look at your airspeeds once stabilised in the cruise. I would hazard a guess that you will be indicating M0.78~M0.80, or about 250KIAS.
(These are approximate figures to illustrate a point. In real life your numbers may be slightly different depending on your operation, your aeroplane, and the prevailing ambient atmospheric conditions)
At FL410 setting 280Kts will put you into an overspeed condition.
At FL410 setting M0.76 will have you very close to Minimum Maneuver Speed.
Whereas setting 280Kts/M0.76 is fine at lower cruise levels it doesn't necessarily work for higher levels. Setting Turbulence N1 (which is usually somewhere about 1% less than your cruise N1) will usually ensure you neither overspeed, or underspeed. Boeing don't differentiate between cruising flight levels, and so don't provide recommended airspeeds.
ps. Don't forget that if you are in RVSM and the turbulence is so severe that you cannot maintain your Flight Level within tolerance you need to advise ATC. Thankfully genuine "Severe Turbulence" is comparatively rare.