I think this is a difficult question to answer, and it may vary with the setting as well as individual institutional protocols. People take short courses of corticosteroid therapy in the outpatient setting with no glucose monitoring, because the course is short (7-10 days, for example), so the adverse effects associated with long term therapy are unlikely to develop and will resolve with the completion of therapy. If the person is diabetic, they are already monitoring blood glucose and will make adjustments to their normal regimen. For the hospitalized patient, I would imagine that CBG monitoring would be protocol. One drug reference I have recommends 2 hour post prandial CBG monitoring. However, at my facility, these patients would be put on our Sliding Scale Insulin Protocol with AC and HS fingersticks, and the type and amount of insulin appropriate for their CBG given. The doctors have some leeway to select a low, medium, or high dose regimen depending upon the individual patient's set of circumstances, like if they have already developed some insulin resistance and may need higher doses of insulin.
It sounds like maybe your doctors make the decision based upon the level of insulin resistance of the individual patient, i.e. a non diabetic patient for whom corticosteroid therapy is new and/or expected to be temporary, vs. a patient who is on chronic corticosteroid therapy and has developed insulin resistance.
You could always ask the pharmacist or the doctor.