BS6SC
You’d think Voldemort would have been smart enough to consider the Sorting Hat. The intelligent thing to do would be to use a Confundus Charm on the hat so it yells out a Hogwarts House as soon as it lands on his head. That’s not unheard of. It happened to Draco Malfoy. Why risk allowing the sorting hat to see into your mind? Even if it’s allegedly sworn to secrecy, what’s stopping someone from confounding the hat to reveal all of its secrets?