There are valuation rules for tax purposes, depending on whether the corporation is a public or private company.  There are also specific rules for accounting purposes.  Tax treatments to both the corporate and employee (as well as election available) would differ, depending on the structure of the plan and, if it's an option, whether it meets the statutory requirements.

These plans should be reviewed thoroughly be the legal counsel as well as tax, compensation, and finance professionals.  Getting these wrong have serious ramifications not only to the employer, who may be penalized for compliance failure, but employees who may be exposed to substantial monetary penalties such as the 20% excise tax applicable to §409A and missed opportunities for tax mitigation such as §83(b) election.

[Edited: Omitted to reference Finance as a key stakeholder and process owner for any equity plan]