Rather safe than sorry, they say. I have gone through veins with blood pushing some of my gear out when I retract the pin, which pisses me off, because where I pinned 500mg Cyp, e.g, I have now lost at least 150mg. I have never drawn blood when aspirating. I have hit nerves in the glutes with quite a fair few compounds and also in the shoulder, mainly with hcg. Hitting a nerve is a whole other type of painfull experience.