It's best to send them in as soon as possible so that your profile is more complete. This will ensure you get interviews earlier, especially those schools that need everything turned in before even considering interviews. This supplement is becoming more and more important, with MCAT scores being another, and it's best to get it turned in so the admissions committee can read it. Please note, there are schools that accept letters up to the deadline and grant interviews before that too. Just make sure you have everything in, in a timely manner.