Hi,
It depends on power supply current.
This device works both from 5V and 3V. For power-saving mode you may use Idle Mode and Power Down mode. In first case, device will eat 5/1 ma (5/3V); in second case - about 100/20 mka. The problem you will reach is that there is only one way to exit from Power Down mode - the hardware reset.
So: if 5 ma due sleep is okay for you then use internal T0/T1 as alarm clock. However, if you need keep much more power then use external reset watchdog and put 89c2051 into Power Down mode when it is not used.
Good days!