[delphi.com.ar]

Si utilizas la TRxDBGrid puedes utilizar el método SelectAll y UnselectAll para todas las filas, en mi versión ampliada de este grid, he agregado el código de InvertSelection que te paso aquí:

procedure TRxDBGrid.SelectAll;
var
  ABookmark: TBookmark;
begin
  if MultiSelect and DataLink.Active then begin
    with Datalink.Dataset do begin
      if (BOF and EOF) then Exit;
      DisableControls;
      try
        ABookmark := GetBookmark;
        try
          First;
          while not EOF do begin
            SelectedRows.CurrentRowSelected := True;
            Next;
          end;
        finally
          try
            GotoBookmark(ABookmark);
          except
          end;
          FreeBookmark(ABookmark);
        end;
      finally
        EnableControls;
      end;
    end;
  end;
end;

procedure TRxDBGrid.UnselectAll;
begin
  if MultiSelect then begin
    SelectedRows.Clear;
    FSelecting := False;
  end;
end;

procedure TFSDBGrid.InvertSelection;
var
  ABookmark: TBookmark;
begin
  if MultiSelect and DataLink.Active then begin
    with Datalink.Dataset do begin
      if (BOF and EOF) then Exit;
      DisableControls;
      try
        ABookmark := GetBookmark;
        try
          First;
          while not EOF do begin
            SelectedRows.CurrentRowSelected := not SelectedRows.CurrentRowSelected;
            Next;
          end;
        finally
          try
            GotoBookmark(ABookmark);
          except
          end;
          FreeBookmark(ABookmark);
        end;
      finally
        EnableControls;
      end;
    end;
  end;
end;