At those flight levels you should stay in MACH and shouldn't worry about MMO overspeed unless:
- You were already cruising at a Mach close to MMO e.g M 0.8 if MMO was M 0.82 or :
-WInd aloft are quite strong and during descent you expect a vertical windshear that wil accelerate your aircraft aerodynamically and increase the Indicated Mach like a significant tailwind decrease ex : At FL370 tailwind 70 Kts and at FL350 tailwind 50 Kts that will certainly reasult in an overspeed .