I utilize parent volunteers all of the time. The only requirement I have is that each parent have a day of the week and choose morning or afternoon of that day to consistantly volunteer. If they can't make it on a particular day I ask that they email or
call me so that I don't have a backlog of books to shelve for the next person. Assigning them to morning or afternoon gives them the flexibility to volunteer an hour or two without being constrained to a specific time. It seems to work for us.