As a general rule when shooting headshots, we recommend framing the subject so that the crop runs just across the top of the hair and just below the shoulders. We also recommend shooting the subject in the center of the frame as well as off to one side, such as the right third of the frame. This should leave enough negative space to allow for multiple applications of the images when used later, perhaps on a website or in a brochure. If your client requests a different look or crop, it will probably be in your best interest to oblige them.
Be careful not to crop below the hairline or above the shoulders. Otherwise, your clients may appear bald, whether or not they actually are, and without a clear neckline and shoulders, you may end up with more of a floating head portrait rather than a professional looking headshot.