Question about light mapping

I don't know if this has been asked before about light mapping, I've watched about a dozen tutorials on the subject but they mostly deal with just having a single building or structure in your landscape, now what I'm wondering is:

Is it best to have all of your structures placed onto your landscape (terrain) before you begin the light mapping process to bake in your shadows?

Or:

Is it by structure dependant and does not matter either way?

I hope I explained that right, I'm new to this stuff and have only been working on my Unity FPS game for about a month.

It wouldn't make sense to place items after lightmapping. Imagine putting a sun-blocking building into a scene, after it was lightmapped. Its cast shadow wouldn't exist.