Even if the flight is known to be late, ALWAYS go to the airport as if it was to be on time, even if you have no bags to check. If you have bags, they do have to be checked in before the deadline of the on time flight. This because of many reasons, including bag scanning system. As mentioned before, you never know they are able to get you on another plane or the delay isn't as long as scheduled...

To go a little off topic:
Not so long time ago, we had a flight out of BRU to LPA which was delayed for over 5 hours. Some travel agency called all their clients (about half of the flight) their flight was late and they could go later. No way, Jose. Half of those folks lost the flight but the agency was willing to pay all the extra costs to reopen CI (big $$$).