The organization I work for provides a code of conduct to help volunteers, staff and board members make ethical decisions in our work. The professional association also has an ethics policy, however when it comes down to it, you are the one that has to have make the decision quite often and you'll have to live with it.
However an ethical dilemma that is often presented is the interest or request for staff to volunteer at our events. Although these staff are not coerced, their is an underlying pressure when everyone else is volunteering, and you aren't. While I obviously understand the need for the right trained person, and sometimes you run short of true volunteers, or you could have extremely well trained and trusted staff take on these roles...the short term gains can have long term losses for the organization. If the employee was unhappy and went after them for unpaid time, or if staff assume they are going to receive lieu time for the time they are putting in for example, the losses or decline in staff morale can be seen. The other issue that is presented when staff volunteer is the removal of roles that volunteers enjoy doing, especially roles that may have included trust or power, as they are often then relegated to less trained positions.
There is a great article by Charity Village titled: Nonprofit staff as volunteers: a risky proposition. Take a read and see the risks you play when staff volunteer for their own organization.