Gear is stored in multi draw vials and ampules for a reason. The less you tamper with it, the better.
Sometimes I'll draw up my shit the night before, if I plan on pinning early in the morning and don't want to fuck around when I'm half asleep and still waking up. Other than that, don't do it... some people draw up peptides and shit before training so they can pin them immediately after, and that's another decent reason. But other than that, you don't want to risk sterility issues just to save a tiny bit of time.