In my experience, it depends on the flavor and the next flavor used. For some flavors like menthol or a particular RY4 I use, you will never get the smell/flavor out of any thing made of poly-pro. I can't help but to feel some of that flavor has to bleed in to the next syringe full. On the other hand, I would go from a french vanilla and then with a quick rinse go to say a butter pecan without worry. When I was using syringes a lot I ordered a bunch of cheepies off of Flee-Bay and and gave most of my popular or particularly strong flavors their own syringe.
For my other lab ware I use a diluted dish soap on anything that had a flavor in it.