I mass mailed last week with cover letter, 1L transcript, and resume. I sent one out last friday, to my top choice who is not at my school's OCI, and was emailed about in interview on Tuesday. I sent the rest out on Tuesday and haven't heard anything back. I would say go with at least cover letter and resume. If a firm doesn't like what it sees from that, it will not waste time reading a writing sample and you'd be out the extra few dimes it cost to mail the additional weight. My two cents.