For this case, since my datagrid only shows a single table, I am just going to store the row count of the datasource to the datagrid's tag property at time of binding.

The reason I don't want to look back to the datatable for row counts is because the datatable gets used for other stuff and therefore changes where the datagrid stays static and just serves as a list.
I ended up doing something like this:

// -- Always set the row count to the tag property when binding the grid
this.grdJobs.SetDataBinding(dsJobs,"Jobs");
this.grdJobs.Tag = dsJobs.Tables[0].Rows.Count ;

// -- Other places in the code will look to the tag property for the row count
int iRowCount = System.Convert.ToInt32(this.grdJobs.Tag) ; 
for (int iRow = 0 ; iRow < iRowCount ; iRow++)
   {
      if  ( (bool)grdJobs[iRow,1] ) 
       {
          // Do something with the items that were checked
       }
    }
            

I surprised that this is not a property.
Thanks to all who helped with this!