Obviously first timer question: Do I leave the expansion gap around all walls, not just the walls that the tongue/groove sides of the wood strips are facing? I had presumed it was all 4 walls, but in a book I noticed in the pictures that they had the board ends butting against the walls.

Also, my neighbor had theirs professionally installed, and the installer didn't have an expansion gap at all on any walls. This is up in Washington where it's not very humid compared to other parts of the country, so I'm wondering if that's standard in non-humid areas.

butt ends are often net fitted where there will be no trim to cover the expansion in areas where moisture swings are not a problem. MY two cents, if you are going to cover it with base why run the chance of not having it when you need it?