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.
I want to thank my wife for a birthday gift last year a D7F Dozer that made this possible we started this 3.11 acre pond one year ago this month we only get to work on it on week ends and evenings I've been monitoring this site it's been very helpful thanks to all. My boy is going to post pictures from the start to now.
I put topsoil back in the basin of the 3 acre pond we built on advice of our NRCS agent. Some of the types of clay that are in our area have a tendency to not settle out of the water and so muddy looking ponds are the norm in our area till enough sediment and plants clear it up. This can take a number of years. Topsoil has more silt in it and less clay, so the larger particles of silt tend to settle out better. I can say for a fact that it worked because I have other ponds this was not done in and the topsoil did help with clearer water. I still have limited visibility, but it is caused by algae rather than clay particles.
This may not apply in your situation at all, so don't think this is a recommendation for you to put top soil back in the basin. I'm just relaying one reason that in certain situations top soil might be beneficial. As previously mentioned, it does add to the nutrient load, which could be a good or bad thing depending on if what you end up with lacks nutrients or has an abundance (from runoff, for instance). So there are a lot of variables with no simple, single answer for all situations.
It is always beneficial to put it on the area that will be growing grass, such as on a dam and its backslope.
As far as depth, deeper is better where you want to grow grass. Just enough to cover the bottom clay for the bowl, if the clay is they type to cause turbidity problems. Two or three inches is enough if you can get it spread that thin successfully.
