I honestly don't see Percy joining with Lord Voldemort.
Percy wants to be relied on, but he also wants power. What kind of power would Voldemort give him? Very little. His role in the Death Eaters would basically be to lick Voldemort's boots and do his dirty work.
He'd have no chance of promotion, because Voldemort wouldn't trust him over the deatheaters that have been loyal to him for years. There's nothing at all in it for Percy.
I think our ambitious little ex-Head Boy will stick with the Ministry, through thick and thin.