I had a shoot today using Canon EOS 5D mkIII and 1DmkIV, where I tagged the keepers in camera and erased all the rest from both cameras.
I ingested tha cards at once from both cameras and only those from the 5D mk3 and all other photos lost their tags.
If it would be a reset tag option then all files would loose their tags. And I didn't set PM to reset tags.