Anytime I ask myself if I should suction or not I've always found that the answer is a strong yes. Worst case scenario is that your patient's sqeaky clean airway is now a little bit more spotless. Ideally the proper answer is to look first for something objective, but as they say, "treat the patient, not the monitor." If someone seems like they have respiratory distress from secretions then they probably have respiratory distress from secretions regardless of what their vital signs say and would probably benefit from suction ASAP.