You should always keep your fingers on the wheel and control the return rate (to neutral), or steering in the other direction past neutral. Grip the wheel so that you can turn left and right naturally, without having to re-adjust your grip. When you let go of the wheel, you lose all control. Watch the fast guys at your track, they drive like the pros drive, fingers always on the wheel, car always in full control.

I should add that if you're letting go of the steering wheel past a corner, you're not really driving the correct line. You "unwind" the steering as you apply the throttle coming out of the corner. Think of the two as connected by an invisible string... the string is always tight and can't stretch. So, you apply more throttle as you straighten out the wheel, in a smooth, combined motion. You can't do that if you let go of the wheel.