You could also use an equal-area projection. HR used Lambert Azimuthal EA, I believe, centered on each tile.

I personally like equal-area cylindrical projections. No wasted pixels. I know a lot of folks were put off by Peters', "Mercator Projection is racist," silliness, you could always call it Gall Projection. Actually, I prefer the Behrmann Projection with standard parallels at 30º N/S, it seems to look less, "continents hanging out to dry."