Workorders should never be deleted. To do so breaks the continuity of the numbering system, and also is a "red flag" in an audit. It signals "missing workorders" and "hidden income."
If work is not completed on an Estimate, it should be noted and then closed, using the 
option in the Workorder Screen. This provides a positive audit trail of all customers, work that is completed, or not, and the reasons. It also satisfies accountability in an audit.