There are problems with both suggestions...
If you don't filter the panes, you can wind up with 100's of entries in the pane, many of which are completely irrelevant, making it tiresome to locate the specific tag or tags you might be trying to target.
Refreshing the panes after each tag change can slow things down considerably for more complex view schemes. I've seen instances here where it could take more than 10 seconds for a tick to appear in the pane.
In situations such as you describe, I switch to using the tag window, where, regardless of selection, all choices are available for list type fields, and auto-complete can help for large lists of 'standard' edit type fields.
Another approach could be to sort the file list by [type], which would keep all the unassigned files together in the list, and so easily selectable, and you would then have all existing options for tagging mode available in the panes above.
Again, this is one of the reasons I created a "Workshop" tree node. All viewschemes in there are set up specifically for work. In there, I have pane, column, sort choices etc. etc. all geared towards helping me get the work done as quickly as possible, leaving the main media type nodes purely for playback pleasure.