I am an employer in Washington state and have purchased a small number of companies over the past several years and am in the process of combining them. One of the companies allows employees to drive company vehicles home each day, while another requires all employees to keep them at the shop over nights and weekends.

Are there any discrimination issues if I were to allow only select employees to drive the vehicles home? If so, is there any particular suggestions on how to accomodate such a 'discretionary' benefit?

Unless you determine who is and who is not allowed this benefit based on race, religion, national origin or another characteristic protected by Federal or state law, it is not illegal discrimination.

You could allow employees in x, y and z positions to drive the vehicles home but not those is a, b and c; you could allow it by department; you could not allow it at all, or any other non-discriminatory factor.