Steve
You probably know this, but just in case here goes—There must be a good foundation on an embankment pond, There cannot be a seem under the dam and a layer of topsoil will allow that to happen. Under the embankment it is wise to remove the topsoil and any organic material, roots, grass, weeds. Over time that stuff will decompose and could cause a problem.
As far as the topsoil on the rest of the site, it is usually, valuable and hard to come by, and can be used anytime, not many people will bury it.
When Elvis Presley built his pond he saved his topsoil.
Otto