I also use a common 'res' directory by pointing all of the album/projects outputs back up to the parent (wrapper) directory. That is I move the generated res directory back one level.
Some people like to use different skins or styles for each sub-album. This implies that there should be a separate res directory for each album. A single res directory works for you, but not for everybody.
Typically, the size of the res directory is fairly small, so multiple res directories would not cause much overhead.