I would be content in most cases with an ultrasound exam alone. It is the most useful test for determining the cause of a murmur and the significance of the murmur. While I am not an expert on ultrasonagraphy it does seem like it requires some expertise in positioning the patient and the probes to obtain a standardized image that can be used to accurately assess the heart's condition. We try to refer to veterinary cardiologists on the theory that most of them, or at least their technicians, have more experience in getting the technical aspects of the imaging done properly. Plus the cardiologist interprets the results more often than most vets, too.
Most cardiologists seem to like to run an electrocardiogram and to have X-rays taken too, though.
I have many clients who would have to be careful making a choice between diagnostic techniques when determining what to do for a pet with a heart murmur. Many veterinary practices have the capability of doing ECGs and X-rays and fewer have the capability of doing ultrasound exams. I have an ECG machine. I almost never use it. While there would be a lot of argument from many vets I think that ECGs do not offer enough information in the case of a murmur to make it a justifiable expense when the cost of diagnostic testing is a factor.