I usually pull all into one syringe and then back fill a freshy if it is an all in one, I know randy uses a lure Lok and replaces the tip. If you are keeping the tops clean before you draw then you shouldn't be contaminating, the important part is after you use a needle four times in a stopper its quite dull. The backfilling into a fresh syrynge or replacement of a tip allows for a smooth like butter injection. If you back fill the barrel just make sure you don't push it out the top when you re insert the plunger.