To me, the etiquette is don't. I too often like keeping my backpack with me during the flight, in which case, it just stays under the seat in front of me. When two people who do not know each other share three seats, there is really no reason why one of them should benefit from 2/3 of that total lateral space.

And I'm with others on thinking there is no obvious 'best seat' bar a few specific configurations (E190, some exit rows, etc) though typically they always come with tradeoffs anyway.