Thanks! Actually, that's not a stupid question at all. Depends on a few things.

My mood.

If I have time or it's a personal project, I try to model it all out of the same mesh.

If I'm in a rush, run into a modeling wall and have a spare part around, then I use a spare part.

Also, sometimes things change after the model's nearly final and I have to create the new part seperately, then attach it later.

So, you see, it all depends.

  In this instance, to keep a bit more control over what I was doing, I built the breasts, then started the ribs, connected them to the breasts, finished the ribs, started the naval/abdomen, connected that to the ribs, finished the abdomen.  :)  So you see, this time, it was a little of both approaches, as niehter part was "completed" without being attached to the rest first.