You can do it in either RowDataBound or PreRender event:

   protected void allGrids_OnRowDataBound(Object sender,
System.Web.UI.WebControls.GridViewRowEventArgs e)
   {
       if (e.Row.RowType == DataControlRowType.DataRow)
       {
            System.Web.UI.WebControls.TableCell cell =
e.Row.Cells[myIndex];
            if (check something for cell)
           {
                  e.Row.(set properties as needed)
           }
       }
   }

Theres an example here

http://msdn2.microsoft.com/en-us/library/aa479342.aspx

Regards

John Timney (MVP)
http://www.johntimney.com
http://www.johntimney.com/blog