Aspiration: Not so critical with peptides, important when injected oil based testosterone and such. Insulin in a vein would be a MAJOR problem. HGH and IGF in veins would not be dangerous, but you might feel it.
SC/SQ injections: when injecting into belly fat, the chances of hitting a vein are very minimal. If you have some blood, you may have nicked a surface vessel and that does not mean that the injection went into the vessel. I do not believe that diabetics are ever told to aspirate with SQ.
IM injections: If you hit a vein, most likely you will go through it and inject below. So if you aspirate clean, you can still bleed or bruise. If you hit a blood vessel, you will probably feel that as the veins have more nerves than the surrounding tissue.
Many think that one should do bilateral IFG injections. But others feel that the amount injected will saturate the available IGF receptors in the body and the net effect will be systemic. In that case, a single injection in the belly fat would provide the same result. We have no clinical research to guide this. The fact that some inject IM and have nice results, proves nothing and might be wishful thinking. But no harm in the IM injections either.