After thorough research, it really depends on the firm.
If it's a major firm that hires 50+ SAs, then they're doing at least like 70-80, maybe 100 CBs. You do not need to send thank you emails, since their system is very regimented. They interview you, submit their evaluation, and forget you existed. Even if your Shakespearean thank you letter brings them to tears, if they are not on the hiring committee, it will do you no good. (at big firms like that, they generally aren't on the hiring committee.) At firms like this (Skadden being the frontrunner), you are a number. A cog. Your feelings don't matter, whether positive or negative.
If it's a mid-size to secondary market major firm that hires 2-15 SAs, then they're doing 5-25 CBs. Definitely send thank you emails. Reasons: 1.) It's a smaller pool of applications, so it's impossible that they will be swamped with thank you's and end up hating them. Their work load is also likely much less overbearing than big NYC firms. 2.) Secondary markets (generally) have more personal and hospitable cultures, such as in the South or Midwest, where courtesy is expected. 3.) Often times, at least 1 of your CB interviewer is on the recruiting committee.
Thank yous are appropriate after lunch where the firm/associate/partner pays, obviously.
Firm receptions is way too gunner and tryhard. The exception is a followup email: "Thanks for taking the time to talk to me. I was wondering about XYZ. etc."