Doesn’t really matter (up/down, left/right, or even somewhat “randomly spaced”), so long as you map it correctly.
Using @wizard’s new tool:
Pixel coordinate mapping tool for images
You can map it as simply as taking a photo and clicking on the LEDs in the right order. (So don’t go too random to the point of you not being sure what the next pixel is, but of course you can always run KITT and see the order)
With most other controllers, keeping them in a straight and even line/order would be important, but with PB, not so much.