What he said.
Stencilled shadows would be awesome, soft shadow possibility even better, and considering there are quite a few mods that involve or are planning to involve atmospheric missions (Shadows of Lylat, 158th Banshee Squadron, probably Diaspora, even retail FreeSpace 2 nebula missions) it wouldn't be
wasted effort. Just possibly excessively awesome considering the implementation of soft shadows would also work if you wanted stenciled shadows, but stencil shadows wouldn't work if you wanted to have soft shadows.
And like you would have noticed if you read my messages thoroughly, the shadow edges on the photographs of our space ships don't show soft shadows, because the spacecrafts are so small and distances so short that it really doesn't start showing on the photographs. Self shadowing of fighter/bomber sized ships would look pretty much like stenciled shadows for obvious reasons, but remember that an Ursa is already larger than the Space Shuttle Orbiter, and we haven't built six kilometres long warships so showing images of Skylab or the Orbiter isn't really relevant on how shadows would look on that kind of behemoths...
Also, regarding perfectionism, I refer you to my recently aquired user title.
I do think that if possible, modeling things as accurately as possible will be more beneficial in the long run, even if technically easier but more crude way would suffice for current purposes. That's why I think that
if feasible, soft shadows wouldn't be wasted effort. If it's not feasible, then stencil shadows are just fine and dandy. But I am not a coder so I wouldn't know how much more difficult it would be to make the shadow geometry realistically defined by the apparent diameter of light source. Probably a lot.
Stencil shadows with ambient occlusion would probably make me defecate ceramic building materials right there, I don't know what soft shadows would do...