… and a couple of rules:
If a parent (summary or roll-up) element is decomposed, it must be decomposed into at least two children elements.

Parent elements must be exactly the sum of their children elements in terms of scope of work, effort, resources, and budget.

… and a few suggestions:
Be sure that the WBS covers all the work to be done on the project, including project management itself and the communication plan.

The PMI® definition suggests that each WBS element should have at least one associated product. This maintains a results- or outcome-oriented approach to the project. Try thinking of activities in terms of their products.