When you are drawing up the 2nd Medication, you want to introduce the needle in the inverted vial. OTHERWISE, you are increasing chance of drawing up air when you've already adjusted your 1st medication. Think about it... sooner or later that medication will touch the needle regardless when you are drawing up the medication. I think you have to think a little deeper and question whether what you've heard is "NOT inverting the needle when you first introduce air to the vials".
The exception is of course the insulin. When you introduce the needle in the vial to introduce air, you wouldn't want to invert and contaminate your needle as it will be introduced into clear insulin next. But it's all speculation. When we rolled the NPH, can anyone insure that some of the NPH is not under the rubber stopper?
If you are inverting the needle into inverted vial to push the air in.... that maybe contraindicated for some medications but I haven't heard of any... Maybe antivenom that you can't shake... bubbles etc.
That's why in the videos I even verbalize while I'm adding air and the vials are sitting on the table.... "Air to Air"!
I usually teach from the viewpoint of the most scrutinizing, the most strict, the most stringent CE so if you cover all bases, no one has to every stop you or question you. I know sometimes I do go overboard but usually tell you the reasoning behind certain actions so you can make your own mind whether or not it sounds like a good idea.