A state machine diagram should always be developed for every class in the domain model class diagram.

The statement "A state machine diagram should always be developed for every class in the domain model class diagram" is not necessarily true.

State machine diagrams are particularly useful when a class in the model has significant dynamic behavior, that is, when an object's state can change over time, usually as a result of events and conditions.

Here are the steps to consider:

1. Identify the classes: The first step in creating a state machine diagram is to identify the classes in the domain model class diagram that have significant dynamic behavior.

2. Define the states: For each of these classes, define the possible states that an object of the class can be in. A state is a condition during the life of an object which it may either hold, satisfy some condition for some time, or wait for some event.

3. Identify the events: Identify the events that can cause a transition from one state to another. An event is an occurrence of a stimulus that can trigger a state transition.

4. Draw the transitions: Draw the transitions from state to state in response to the events. A transition is a relationship between two states indicating that an object in the first state will enter the second state if the specified event occurs and the specified condition is satisfied.

5. Review and refine: Review the state machine diagram for accuracy and completeness, and refine it as necessary.

In conclusion, while state machine diagrams can be very useful for modeling dynamic behavior, they are not necessary for every class in the domain model class diagram. They are typically used for classes where the behavior of the objects is complex and needs to be clearly understood.