Its a matter of choice i think, at least for me, I would drape the cloth if its way off in fitting the character or if you have an animation where the cloth is not linked to the interaction object. You have to remember that drape is done before the actual simulation is calculated and are not stored within the simulation, so if you were to cancel the calculation during the drape the cloth would return to its default starting position. Here is a small test to show it.

At the top is the actual scene.

Next image shows what the first frame using drape looks like.

Then the final result for both a simulation using drape and one that is not. As you can see the different is not that huge, however if the fish for instant were swimming forward from frame 1, using drape would allow the cloth to interact with the fish before it actually moves and therefore move with it during the animation.

So in such case using drape would be a good idea, otherwise the fish might have moved before the cloth reaches it. If you are simulating a piece of cloth for a human it would normally be linked to it and therefore follow along with it, so it would probably not make a huge different whether you drape first or not, at least I wouldn't bother doing it in such case, because the actual simulation would make it look realistic enough in end anyway.