I think you are wrong Heavenfall. It doesn't matter what kind or how many items you use, because it will only be visible when you zoom way close and even then, it is only displaying a small portion of your town. This is some I would like to test in the future though.
Yes, it does matter. Whether or not it's being show, it's still in memory. And regardless of how many of -your- objects are on screen, it still has to play nice with every other object on screen. And you have no idea how demanding the scene will be. So the cleaner your object is, the better it'll work in tandem with everything else.
For an extreme example, imagine putting a gold coin on a table under a shack. Due to the shack, at no point will the table or coin be visible. The game still has to keep those objects in memory if only to cull them from view.