In the top-left corner of the screen there's a menu of main filters, letting you switch between seeing all bookmarks, or a combination of read, unread, public, private, etc... In using the application I quickly realised that it would be handy to have bookmark counts in that menu.
So I added that:
These counts are a little different from those in the tags list, in that they always show the number of matching bookmarks amongst all recorded bookmarks, not just those currently on display (and so subject to any sort of filter that's in play).
I think this is the right approach here. I know for sure that that's what I want from this, and I am writing this for me after all...
What's handy about this is that it makes it easier for me to see how many bookmarks haven't been tagged, and also how many I haven't looked back over and marked as read. This is already helping me get to untagged-zero.