One of the nifty things about drop bars is that as you move your hands around, you also vary the angle of your upper body.
Basically, you should never be able to really find that "one" position. With any comfortable stance, the longer you are in it, the less comfortable you will be. So, you vary grip not only for your hands and forearms, but also for your back, shoulders, etc.
With a flat bar, the only way to vary your "riding geometry" is to flex at the elbow to raise up or down. Obviously something that cannot be comfortably maintained.
Another thought about the elbows is this; While most of the time you won't lock them, sometimes you will, to give your shoulders or triceps a bit of a rest. And, though you may be "breaking a rule", there's nothing wrong with that.
I guess the real answer is that there's no single answer. Your bike offers you many solutions, which you can utilize as one becomes tiring/uncomfortable. Think of it as avoiding repetitive stress strains or cramps.

Happy riding!