IV Incompatibility

I'm currently taking NCE on IV Therapy and was puzzled by the instructors discussion on drug compatibility. I don't normally work with patients who require IV treatment but I took the course to give me an opportunity to apply at an outpatient facility that does require additional training/experience in IV therapy. My question is, if I'm supposed to give an antibiotic by IV, and I've checked that it is not compatible with the fluids that are currently running, how would I give the patient the medication? My rationale is to call the pharmacy if I'm uncertain but my instructor said that when in doubt about drug compatibility, it would be ok to flush the I.V. administration set with NaCl before and after medications are infused. Is that correct?