it's completely optional ... but, you don't get any need based scholarships for sure if you dont' give it to them.
The messed up thing is, when you sign up for LSAC, they say its optional. SO... i didn't put it in. BUT ... then like 2 weeks after the test, I got a letter saying my SIN number was wrong and that if I wanted to get my score on time, I had to fax it to them ASAP. stupid LSAC.