Other than to make a big batch of dubbing I see no reason to clip or shave an entire mask. When you do this you lose the option of choosing from all the color variations available to you. You really have colors from darker brown to white. If you really want to clean off the hide you should at least blend up batches of various shades. And don't forget to try using wet blending as the results are a more thorough mix and is much easier to store and handle.