You should have your LSAT score well before the ED deadline. At NU, it is 12/01. The interviewing period ends on 11/15. You should have your LSAT score for October by the end of October.
Wait until you have that score to decide what to do. NU accepts the higher of two scores. If you get a 171-172, then I wouldn't apply ED. If you pull off a 168-170, then I would do it if it's your number-one choice. Applying ED has advantages and disadvantages. Applying ED will not HURT your chances at getting accepted - it will HELP your chances at getting accepted. It WILL hurt your chances of getting offered merit aid.
Best strategy: kill the LSAT, wait for the score, get the app in early regardless of how you choose to proceed.