As I said earlier I only add air to vials I am drawing finished doses from. When I move reconstituted liquid into a fresh empty vial I just vent it so the volume can go in without resistance. Otherwise it can be hard to pull the liquid into the syringe. Someone else pointed this out before as well.
The earlier comment was about using the same needle on a pen for several injections. There is no reason to take that chance when needles are cheap, though some still do it. I supplied a friend with a pen and extra needles and reminded him to swap each time, but after four weeks he still had several left and reported no trouble.