"Let your Yes be Yes, and your No be No" or something to that effect, from the NT.
Many these days have trouble living up to that, lots of "I'll be there" but little follow-through, showing up, late or on time, prepared or not.
I try to let people know when I will be late or not there for rehearsals, services, etc.
My take - signed contracts will have little or no effect on those who do not understand the concept of Covenant...