Warning: pure conjecture ahead.
I sent mine today. I know that programs certify their rank lists weeks before the deadline to avoid any website problems, and with the last interviews nationally occuring last week and just a few stragglers this week, I would imagine most programs are holding their ranking meetings very soon, probably this week. You would obviously want to send your letter before your top program's residency selection committee meets. I guess the dilemma then becomes when to time your letter for maximum impact. I doubt that it's detrimental to send it early - programs are usually pretty good about keeping communication from you in your file, and I think what we perceive as the emotional impact of a well-timed letter is probably overrated for these busy programs that receive lots of communication from a large applicant pool.
SO, my advice is to go ahead and send your #1 email. I think it can only help you, and you don't want to miss the boat.