To answer the question more directly, I often receive new clients in the following situations:
Often in the case of back pain, stress related illness etc. Clients feel they have exhausted all that formal medicine has to offer. This is ideal from my point of view as a full medical history has been built up and the individual's condition has been pre-checked by a medical practitioner.
Many of my pregnant clients and moms with infants and children are receiving formal medical care for their conditions and related complications. They typically continue with this care as they attend CranioSacral Therapy to further relieve symptoms or to reach the parts that they feel formal medical interventions cannot reach.
In summary, in my experience the answer is yes - CranioSacral Therapy can be used alongside formal medical interventions.