Personal experience. If you want to file the N-400 do it. No worries on late filing, however, it does seem to hasten the I751 process (not that it matters in the end - just nice to have it done.) We sit on the I 751 for nearly 2 years - filed the N400 and 6 or 7 months later - both are scheduled, approved and my wife is in queue awaiting oath ceremony. I believe that if we had filed as soon as it was clear to do so, she would already be a citizen, and it would have saved us needless months of waiting on the I 751.