This may sound strange if you've never thought about it.............
I like a shallow mallet, BUT.......after finding one that is pleasing to the eye, weighted right, and feels good, IT MUST NOT TWIST WHEN DROPPED ON THE LINE BEHIND THE BALL!
In other words, I will line up a putt, address the ball and then loosen my grip so much that my putter head falls to the ground. I still have the grip in my hands, but very loosely. From this point I retake my grip at the point where the putter will be just above the surface of the green when I swing. If when I initially 'drop' the putter the head twists, I am completely out of my routine. It amazes me how many putters (and even woods) will do this. Try it next time you are at the pro shop!
Ron