Suppose a firm estimates its WACC to be 10%. Should the WACC be used to evaluate all of its potential projects, even if they vary in risk? If not, what might be "reasonable" costs of capital for average-, high-, and low-risk projects?

The WACC will be 10% for average risk

below when the risk is low

and above 10% when the risk is higher than average

as the cost of capital (required return from the stockholders) will increase pushing the WACC higher

Explanation:

As the WACC is composed by the cost of debt and the cost of equity a higher risk will require a better return for the investor thus, the equity proportion that determinates the WACC will change along the project risk.