Taxation impacts the BEFORE tax rate of return on investments.  The rate of return on investment can be highly impacted by taxation.  Short example: Sometimes the rate of return on an IRA, municipal bond, and a Roth can be equal after tax even when the before tax rates of return are different.  Taxes and Business Strategy is the name of a course taught by Bob Wyndelts while at ASU.  The course is a financial planning and modeling class with taxation included in the modeling.