When you choose Verify Tax Status from the Process Button when viewing a datagrid, the options to select tagged rows will disappear. This is a visual indication that the feature is only working against the rows selected at the time you ran the process.
The feature was initially intended to work on the a tagged subset, but the functionality was never implemented. It does work on highlighted rows.
It will bring up a separate box for each tax idcode found so you can make your decisions. Once each record is accepted or rejected it will present you with the next idcode until it finishes.
Selecting multiple rows and running the verification saves you a little time, but moving from one grantee to another and clicking the verify box is almost as fast and this method does allow you notice which records are missed because of an invalid tax id.