If you interviewed with an alumnus, there’s nothing wrong with sending a thank-you note (even though it might come off as a little kiss-ass). If you interviewed with an adcomm, that’s their job.
It doesn’t matter if it’s their job, you still send a thank you note. It isn’t difficult to do. For a lot of people, it won’t matter. For some, it will mean something.
Just make sure to proofread it, and don’t write anything awkward or weird.
I generally use an email unless it’s someone really important (like a judge). For adcomms, I think an email suffices. For alumni, email will generally suffice. If it’s someone you’re really impressed by (like a top lawyer in a market you want to practice in), you can send a mailed letter if you choose.
I’ve found that people in the baby boomer and older generations are more likely to be impressed by a hand-written letter.
Basically, an email generally suffices if you’re going to send a thank you letter.
I think this also depends on market to some degree. I’m Southern. I think in big markets like NYC, thank you notes aren’t necessary. Down here in the South, it’s generally a good idea to send them.