It took about 24 hours of work spanning six days, but I recently made an Excel spreadsheet documenting all the post offices I've ever visited. Including experience notes! (Such as: beautiful office; friendly/nasty Postmaster/clerk; exceptional postmark; arrived just before closing; etc.)
For the most part it was easy to keep track of where I've been, because I've taken photographs of most post offices I've visited (including all since late 2009); and the postmarks in my collection tell me what day I visited any office!
There is the issue of "postmark prehistory", however: what post offices did I visit in my younger years? My father took us [mom and I] to various offices while we were on vacation. My threshold for visiting is thus: I explicitly recall visiting a particular office (with my memory possibly jolted by a postmark I see in our boxes); or I had an interaction with the facility once I started venturing out on my own. That is, I took a photograph of the facility or obtained the postmark (or left a note and materials if it was closed, requesting such); or both.
Every post office I've visited since 2008 is explicitly documented, though it's possible I will be adding more office visits from my younger years. Nonetheless, for the time being I've established the 1,500th post office that I've visited: Atlantic Beach, New York, at the southeast corner of Nassau County.
(If I remember more, 1,500 could migrate to Long Beach, NY, or Island Park; Oceanside; or other offices I visited on Friday the 19th. But let's stick with this.)
The Atlantic Beach post office is, unfortunately, characterless in terms of signage; but it did have a nice four-bar (one of about four I've seen on Long Island) and a black dater with "New York" written out. Class act!
This is the Atlantic Beach Post Office:
Perhaps the most photogenic office on my Nov. 19th "run" was that in Freeport, NY: a beautiful WPA (Depression-era) office complete with vertical murals inside. I took this photo before sunset, and the light struck the façade beautifully:
Unfortunately, that afternoon I also experienced two troublesome offices in terms of documenting their murals for the PMCC Museum. One Postmaster claimed that the authority to allow picture-taking was stripped from him "after 9/11" -- which is false. He insisted that I get written permission from the Long Island District office (which I then got later that afternoon). So, if I'm ever again in the area for some reason I can go back and take a photo. Another insisted I schedule an appointment so I wouldn't interfere with/get customers in my photograph. Which is a legitimate thing to stipulate, though it would be an absolute pain in the butt because that's a 45-minute drive from where I live.
Fortunately, after discussing this for a while at that second office, the customer line whittled away and I was allowed to take photos -- once I gave my driver's license to the supervisor, who then wrote down my license number and held onto it while I took my photos, until I let him inspect every photograph I took of the murals inside the office. Oy.
So once again, from my experiences I must declare the New York metropolitan area to be the least conducive to collectors in the country. Although there are competent people at higher levels in each of the District offices, where it really counts.