Black-Scholes may not be the best model for stock option expenses. CFO.com article proposes that binomial stock option value modeling may be more accurate than Black-Scholes, however, it may also be more easily manipulated by management to lower reported stock option expenses. Additional FASB usage guidelines may be needed to establish allowable assumptions for the new model.

...the traditional method for valuing options, the Black-Scholes model, seems likely to be eclipsed for purposes of expensing employee grants by an alternative known as the binomial method (also known as the Cox, Ross, Rubinstein model).

Mark Rubinstein, a finance professor at the University of California at Berkeley who helped create the binomial model. Rubinstein says the first question he gets from managers is, "Can you tell me how I can get lower numbers?"