The grid is aligned to the chip origin, as far as I know. However, in the final version of SkyWater's density rules for sky130, they calculate metal densities over the full chip and only do the stepped window for FOM (diffusion) density. Our pattern generation does not look at the existing density, so it will apply patterns to any empty area. So you can't, for example, cover half of the chip with a solid sheet of metal and expect it to pass with 50% density.
There is a stress rule for metal over (I think?) 30um which requires slotting, although I also think SkyWater applies slotting automatically, so the rule isn't checked, as it is always correct by design.