No one that I know of places it under the muscle. They are often placed within the muscle. Yes, it is best to discuss with your doctor you preferred location.
You, as the patient, absolutely have the option and the decision. Find a plastic surgeon with those qualities and expertise to explain all the pros and cons of every technique and then you can make a good decision
It sounds as though you have already done your research and answered your own question. If your doctor is not already recommending sub or intra-muscular placement then chances are s/he is not comfortable with that technique...enough said. Glad to help.
You should definitely have your butt implants placed intramuscularly or you will risk feeling the edge of your implant especially laterally. Please see example of buttock implants below.