In the interests of not having to go back and redo anything, I have a couple of basic questions regarding modeling that might seem pretty newbish, but I hope you'll forgive me.

When adding detail (greebling), or faced with a problematic joining of two objects, is it better to leave the objects separate with less total polies, or to go ahead and try to make one object per detail level with more polies?
Also, must every object be an enclosed 3-d affair, or can you have gaps and openings? I guess what I'm asking is if you can use a plane or if you must make a plane a very, very thin box.