According to officially accredited driving instructors, letting the wheel slide through your grip is absolutely unsafe and completely improper. But according to absolutely everyone else, it's totally fine.
Driving instructors aren't wrong, they're just stuck in a really difficult profession. In addition to proper steering techniques, they also have to teach that it's extremely dangerous to start the engine before all seat belts are secured, and that it's possibly fatal to get into any vehicle without doing a circle-check to make sure all tires are inflated and all exterior lighting is intact.
And they do that for a reason. Driving instructors are paid take people who have never touched a steering wheel before, and turn them into fully competent safe drivers within a few dozen hours of practical instruction. Trust me, that's not a job that just anybody can do. Driving instructors all have the technical skills of a Samurai, the reflexes of a Jedi knight, the patience of a saint, and (even the females) balls of solid steel.
But the instructors are not wrong. If you ever happen to be that one person in the world who is coming out of a turn and experiences two simultaneous tire blowouts, while driving on wet ice over a pot hole on a 60-degree grade, you'll wish you were firmly holding the wheel with both hands.