Expanding the perimeter by one tile in cardinal directions helps get rid of the single-tile "protrusions" seen on the far left side of the previous screenshot (those tend to be awkward/more likely to require a guess, which is less-interesting gameplay).
Still need to think about what all this means for coordinates when playing with TTS (don't think I'll do anything special, but there's a big amount of "wasted" coordinate space in the top left and bottom left).