Hm, I would think when you're at least in the fleet or on a deployment that you'd fly with the same guy, so you develop a sense of understanding and trust in the other.
But then again, its kinda cool that you'd get to meet all your squadron mates in that setting.
You've been watching
Top Gun again, haven't you?
There are many many many factors that go into who flies with whom in any multi-crew aircraft, and they can't be met if you're always flying with the same dude.
Don't know about Cobras, but in most Navy squadrons you'll have someone you probably fly with more frequently than others. Usually it's policy for the FNG's to fly with the skipper, XO, or one of the dept heads, both so the New Guy can learn and the senior guy can keep him from doing something stupid. The more senior you get, the more junior your co-pilot, WSO, etc, will get, until you reach the point where you're both about equal...and the Safety Center will tell you that the most dangerous crew to have is two mid-tour LTs.
Then suddenly one day you're the Salty Dog saying, "The fuck are you doing, New Guy?"