If the grid is created through dhxGridPanel, there is no need in any additional steps because dhtmlxgrid will support all layout behaviors:

      new Ext.TabPanel({
              region:'center',
              deferredRender:false,
              activeTab:0,
              items:[new Ext.grid.dhxGridPanel({
                      store: new Ext.data.Store({
                          reader: new Ext.data.ArrayReader({}, [
                          {name: 'company'},
                          {name: 'price', type: 'float'},
                          {name: 'change', type: 'float'},
                          {name: 'pctChange', type: 'float'},
                          {name: 'lastChange', type: 'date', dateFormat: 'n/j h:ia'}
                      ]),
                      data: [
                          ['3m Co',71.72,0.02,0.03,'9/1 12:00am'],
                          ['Alcoa Inc',29.01,0.42,1.47,'9/1 12:00am'],