I, too, have sub-directories of charts of different areas. And would just go into the chart selection box and add the ones I wanted and remove those I didn't want.

But just this morning I decided to take a look at Opencpn's 'chart groups' and see how that works. Sorta slick from a quick dabble at it. You can have a whole directory of charts available and then make several chart groups of selected charts. Only one group at a time is visible, so you can control the clutter on the screen and switch groups at a push of a button.  Anybody played with this feature and can add hints or warnings?