I was in the same quandry. I filed the EAD extension, even though my eventual interview was approved.
If you have a valid H1B (and you haven't changed jobs) then wait, and see if you are approved at the interview.
If you no longer have a valid H1B (or if you've used AC21), then you'll need to file for your EAD extension before the interview, just in case you are not approved on the day of the interview for any reason. (ie name check pending, additional info needed) because to get an i-EAD at the local office your application must be pending for a minimum of 90 days.
If your interview does not go well, and you file your EAD application the same day (2/24/05) 90 days after that is 5/24/05, and you EAD expires on 4/18/05, you wouldn't be able to work for the period between 4/18/05-5/24/05.
Better not to take the risk. File your EAD application at a minimum before 1/18/05. (i.e. before the end of this week!)