After cleaning up the source files for my episode, deleting lots of unnecessary and redundant files, I'm now going through and rendering check frames for each shot again, as thumbnails, to make sure they still work (i.e. that I haven't accidentally deleted required Blender files).

So far, so good (but I'm not far along yet).

Also, the 25% size images will be good for my storyboard & linking documentation (for each scene, I have a documentation page that tells how the files are related to each other).