For a traditional homogeneous application it is sufficient to test the application with unit tests and system tests.
Testing all the services in a SOA landscape in isolation will not ensure that the services will work correctly in composition. Compositional problems can be caused by incorrect assumptions between services. Integration tests can be used to filter out these incorrect assumptions. I recommend adapting integration tests in a early phase in the development cycle to prevent situations in which the cause detection is time consuming and the problem solving even more.
The use of mock-up services will help you to build the integrations tests. I also advice to add the integration tests in the build proces, so that possible problems are detected in an early stage.