I don’t like that choice: you need a 13x13 board to play a 9x9 game and a whole board to play a 13x13 game.
In my goban I chose to place the cuts exactly in the middle between two grid lines.
This way I think I’ll be able to play on the 9x9 piece even though with no extra room for stones.
I just realised that I haven’t said yet that my 4 pieces board will have 4 different pieces: a 9x9, a 10x10 and two 9x10 with different sizes due to the not perfectly square grid.
As I was saying, cuts will be equally distant from grid lines since I noticed that “suboptimal” cuts 
may interfere with stones placement when they’re too close to the grid.
In the cheap foldable board that I bought online, hinges aren’t perfectly aligned and putting stones on the center line is a pain.
So I decided to put the cuts as far as possible from lines.
On a well built machine-cut board this probably isn’t an issue. But I’m pretty sure my board won’t stay perfectly flat!
Well… is that a missing line???
Yep, that is!
On a brown board with black lines it’s barely visible. But on a dark board…
