It really depends on the object, it's importance and what you're trying to do. It's not always about saving a few polygons, sometimes its about UV layout optimization, shading or a better workflow. For example, the the UV layout in the top pic would be a giant quad with a big deadzone in the middle, that uv space is…
If is a hard surface piece like those it is okay to just have one sitting over the order and save a couple of triangles by not welding it together, in most cases. Sometimes you might want to make it all one piece, for example if the angle in between both piece is higher than 90 degrees. That way you can keep those UVs…