When tagging a lot of pictures, it would be nice to have some sort of indication as to whether a parent tag has some selected tags underneath or not. I would suggest that you could improve this by highlighting parent tags (in bold) when a child tag has been selected, no matter how deeply down. The reason for this is that I often have to collapse the tage lists to make it possible to select tags from other parts. I have tried to visualise it with a manipulated screenshot.

Not selected:

Selected:

The tag that is selected is not highlighted, to be able to tell the difference between a selected parent tag with or without selected children. A parent tag should always (and only) be highlighted if one or more child tags have been selected, whether the parent tag is collapsed or not.

By doing this, it becomes much easier to find "runaway" tick-marks and understand why a certain picture is included or not.