Our units' protocol is to decrease the MSO4 gtt. if respirations drop below 8/minute for comfort care patients. Think about it, we often set ventilator rates at 8-10 breaths/minute; even at 8 breaths /min. the pt. is breathing an average of every 7.5 seconds which is not too slow for someone at rest.
I work CV-ICU and while we rarely have comfort care patients, we have had a number of them this past winter so I really think that MSO4 drips are very humane when used properly. I don't turn them off if the patients rate drops to less than 8 breaths /min, I prefer to turn them down instead as it is easier to titrate the comfort level. And of course it is important to monitor them closely.
In the example you gave, your patient is comfortable. What is wrong with that? Continue to monitor closely but to turn off the drip may increase their pain if you are called away from the bedside (to care for your other patients, maybe???) longer than you expect. If you are concerned about the respiratory rate, you could try to taper down 1 mg/hr and observe the effects.
I do think that you should have some sort of unit or hospital protocol to go by though so that all of the nurses are following the same care plan for the patient-- it would cut out those "discussions" you mentioned!