I also shoot with a high thumb on 1911's.
On my USP's, however, I extend my thumb below the safety/decock lever as sufficient downward pressure on the multi-purpose lever could potentially decock the pistol. Also, the safety/decock lever is higher than the safety on the 1911, and attempting to "ride" my thumb on the lever moves the web of my average sized hand away from the backstrap, weakening my grip. YMMV.