Now, think about and rationalize this out. You are taught that every medication has a certain safe rate that you may push it into the vein. The only way you can control that rate of infusion is when
you push it into the vein yourself and are controlling the "push", both the medication
and the flush solution. Whether it's morphine or some other drug that is also compatible with saline, shooting it into the IV line and letting the IV fluid flush it into the vein is not responsible. How are you controlling the rate of injection that way? What if the IV is running at 50cc/ hr? Or, 200cc/hr? Mark my words. . .this may come up on a NCLEX question, so you need to know the rationale behind this. As good practice, you should flush the line before and after the medication, no matter what it is, to insure, for the patient's safety, that it is being given properly.
And, by the way,

this idea that there is a different way, better way or secret shortcuts to do procedures that magically is at your disposal when you are finished with school is a bunch of hogwash. There are right ways, wrong ways and sloppy ways. If there were acceptable shortcuts, your instructors would be showing them to you. Make a decision right now as to how you are going to practice as a real nurse. It's sometimes hard to keep your integrity and follow the good techniques we were taught in nursing school while others around us are making up their own shortcuts and little deviations from the rules to get done quicker. Who's the winner? Not the patient. We're supposed to be advocates for the patients. I'm for quality and the most excellent patient care I can give, how about you?