An e-mail, if anything, is fine. It isn't going to make you, but it CAN break you if you screw something up.
I'm going to a large law firm upon graduation and summered at 2 large firms while in school. I've spoken to recruiters and they both said that while it's nice to get a thank you, it will never secure any type of offer, but it can definitely hurt a student if they make a mistake in the thank you note.