It depends from the movie.
Sometimes these subtitles are separate, and are called "forced subtitles".
In some movies (not all) these "forced subtitles" are on a separate track, so it is possible remove all but these subtitles. It is possible know what of these various subtitle are forced ones watching the movie on a DVD player software such as PowerDVD and see what subtitle track is automatically selected.
However, because of their very small sizes, leaving subtitles don't change so much video quality or compression ratio.