So not so much what your injecting into being the issue, more so the actual bottle of SC. Keep in mind there are air born bacteria such as you see growth on your bread or things you put in the fridge etc. So what happens is you mix your growth with SC for example. Not a problem at all because you use the growth so fast, in fact, I'll not worry and mix 5 or 6 bottles of growth at once knowing it will take me maybe 10 days to use all. Never had a problem there.
So the larger concern is your 10 or 30 ml bottle of SC. You put a sterile needle in when drawing but there are still air born bacteria everywhere , always. Point is, you can introduce that bacteria into the SC bottle and begin to grow bacteria with no alc content. In fact most often you will notice, SC is really a one time use bottle for just that reason.

So here's my experience with it, I understand even the bacteria mostly will not make you sick such as most molds on food will not. Even so, you have to achieve x parts per million for most bacteria to even reach a place by where it could make you sick. What's worked for me with is I try not to keep those bottles more than a month. I know that's far outside protoco? But I've never had an issue doing so and talking thousands of pins over time. I won't even encourage that practice as bac water far superior in the "safe" aspect but often during bac shortages we settle for SC and I'm simplh saying what I've done and had no issure whatsoever with it. Maybe most important, always keep clean technique in your swabbing and environment for pinning etc. As to what you do, that's entirely on you, just sharing my personal use of it and again, I'll always opt BSW if available.