Well that comes down to your own ideals and beliefs really, I presume you saw a dogfight between 2 pilots, the enemy pilot having a higher energy state than your ally and decided to attack the enemy pilot, that´s what I think is justified.
Kill credit goes to you.
However if the enemy is in a lower state of energy I´d sit back and get myself a better energy status first, and probably your ally will attack the enemy on it´s own and maybe finish him off.
Hanging on someone´s tail, or shoulder shooting and waiting till your ally or friend makes a mistake or leaves room for you to shoot is not correct, in my world at least. You put yourself in unnecessary danger as well.
It´s also a more difficult matter than it was in real life, you probably knew your ally or friend and could communicate with him, or when you do make a hastely attack you could always buy him all his beers for the rest of the month, or get a whack in the face
CoD has a (more) rewarding system though than IL2, at least you get some credit, that makes the gap closer from Real life I think.
Homuth