Image A is plenty of geometry. You could remove 2 polys in the wall and still keep the same shape. You could also reduce the frame and door to the same resolution and be fine. If you can achieve the same shape with 6 polys then there's no reason to use 12 or 60 or any more than 6.

There is no rule on poly density. Its however much your system can handle. Higher density requires more system resources.