I'd think that he pure saber skill stalemated, if not declined slightly, upon leaving his place as apprentice. Think about it. It's stated that Sheev hated saber combat and thought the sith were above it. He only used a lightsaber as a tool against the jedi.
Then he became Chancellor and never got to utilize the skill he trained rigorously under Plagueis aside from a few isolated moments. Remember how joyful and excited he was to fight Maul and Savage in TCW? I'd be too after never being able to practice due to being a corrupt politician.
Despite that, his power-growth in the force since TPM probably compensates for any "rustiness" in his saber technique.