I did my thank you notes via email, sent to everyone who actually interviewed came. Then, when it came time to certify my ROL, I sent my top choices love letters. My #1 choice get a very enthusiastic love letter saying I will rank them as #1 and exactly why, the others in the top three got nice love letters saying they've been ranked very high and it would really be a pleasure to match with them. I got my #1 top choice. I think you can do thank you notes however you like.