My 2 cents worth...
Cent 1 - On the interview trail, there are some people who stand out. These people don't need to send that email telling their #1 they want to be there, as they'll be ranked near the top. That said, these standouts probably only make up 25% (if that) of the pool. The remainder are basically tossed into a pool and just arranged without too much thought, apart from some small trivial detail. If you fall here, this is where it may help you. Programs usually go 5-10 deep per spot, so being bumped from 40 to 35 may make the difference at a place with 6 or 8 residents per year. And yes, there is something to be said about picking people who want to be there as their number one choice, as opposed to people who are indifferent or have the place be their 3 or 4 or 10 or whatever.
Cent 2 - PD's do talk to each other, especially if the programs are in the same city/state/proximity. When I was on the interview trail, at least half the PD's/Chairs of the places I interviewed at knew my home program PD and Chair and were good friends (played golf, worked together on projects, vacationed together, hung out at conferences, etc). Does this influence the match heavily? Not sure. Can it? Probably. Worst case, you can screw it up for the students who will be applying next year. Someone from the class before us told 3 surg programs they were the #1. He went to his #1. The PD from one of the #1's he didn't match at called our Dean and angrily informed him that if this happens to him again, he won't interview our students. Plus, who knows where you'll be looking for a job or fellowship at down the road.
That said, tell your #1 they're your #1, and tell the rest that you like their program and that you'll rank them highly, or that they're at the top of your list (this does not mean #1).
Good luck in a few weeks!!!