My wife's 20 gauge 870 is topped off with #3 Buck, because that is all I can find for it. My 12 gauge 870 has 2 rounds of #4 buck followed by 6 rounds of 00, and here is my reasoning:
I live in a fairly small apartment, there won't be a shot longer than 20 feet and over penetration is a realistic threat in several (but not all) directions. Should all of the internet talk about nothing smaller than #1 working prove to be true there are still a whole bunch of pellets inside of whatever decided to come in un-invited and I have made my way to the bigger pellets.
The #3 and #4 buckshot are by Federal, the 00 is Hornady TAP.