Our general advice is to set exposures after building.
This way, you can see what the sims like on the slate before adding your thoughts, which is a valuable point of research.
That said, if you’re certain about particular stands or fades you want from the very beginning, you can set those exposures ahead of time to make sure they show up in your first pool.
If you can’t reach your desired exposures after building, set the exposures and rebuild lineups. The exposures carry into the new build. Rebuilding is a normal part of the process, so don't treat the first build as your one chance to get it right.