The Equal Employment Opportunity Commission has issued a question-and-answer guide, entitled Religious Garb and Grooming in the Workplace; Rights and Responsibilities, which explains the prohibition of religious discrimination in Title VII of the Civil Rights Act of 1964 and provides guidance regarding the types of reasonable accommodation which must be provided to the sincerely held religious beliefs of job applicants and employees.

The guidance states, among other things, that customer preference is not a defense to a claim of religious discrimination and that, as a general rule, an employer cannot lawfully require an employee to cover his or her religious garb, marking, or article of faith if doing so would violate that’s employee’s religious beliefs. 

http://www.eeoc.gov/eeoc/publications/qa_religious_garb_grooming.cfm