Hmmm, you can still cheat off on DB work too. How many times have you seen a BB come up cock eyed in a bench or curl or even a DL? Truthfully, I think that with a DB you could at least address that issue by only training to your weakest side's ability. Nice slow rows/presses concentrating on remaining "level" and when the weaker side gives you stop. Same could be done with a BB. On a BB you can cheat off a little (weak side comes up first, pushing more weight on the stronger side... like moving furniture up the stairs, you want your stronger guy at the bottom because he will hold more load. I think most people assume the opposite).

Anyway, as I stated before, asymmetry is not necessarily tied to strength. And to that end I am not sure that any amount of form in training will undo what genetics have done.