At the risk of sounding harsh, not writing a cover letter for a firm I think conveys that you didn't care enough to research their firm or take 15 minutes to write a simple letter. I think it would seem like you just blindly sent your resume out to any and all firms, as opposed to being actually interested in that particular firm. Unless you are at the very top of the class, I think you do yourself a big disservice by not writing cover letters. These employers sift thru thousands of resumes - you need to give them a reason to take note of you that is more than just a GPA number, and a cover letter is pretty much your only opportunity to do that.