Airbus has a 'soft altitude mode' which ALLOWS vertical changes of +/- 50’ to control speed excursions rather than hosing the engines up and down via the autothrust. This is allowed in RVSM and saves fuel:
".......The soft altitude mode corrects minor deviations from the Mach target by allowing a ±50 ft variation from the CRZ FL. This feature improves fuel efficiency and passenger comfort and minimizes the changes in thrust......"
Why would you want to hold your altitude rigidly in turbulence? - surely more stress on the airframe?