On top of the dam and backside of it... I would. It will get grass growing that much better/sooner.
Inside the pond basin, that might depend on how long it may take to fill up. What would be your guess on that?
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Edit: What I mean by that is, if you are going to leave it idle for a year (like keep the pond drained), it would be good to get some grass growing in there to avoid erosion, and thus some top soil to get the grass growing. However, the top soil will only add to the nutrient load when the pond fills up, and that's not a good thing either. So I wouldn't do it inside the basin unless you are looking at a long term bare soil situation.
Edit: I started grass down the inside slope of the pond to about a foot below where I expected the water level to top of at.
