I couldn't find any info on this topic, so I decided to share it in case someone finds it useful.
I wanted to figure out a way to design my level around the blockmap - the 128 mp grid that Doom uses for collision detection. I was disappointed when I found out it wasn't as easy as setting my grid size in Doom Builder to 128 mp. Below you can see my first attempt at creating a 128 x 128 square aligned with the blockmap:
Aligned with the editor grid, but not with the blockmap.
After a bit of testing, I discovered that the blockmap always had an intersection exactly 8 units south and 8 units west of the bottom-left most corner of the map. By creating a dummy sector 8 units north and 8 units east of an intersection to the southwest of my square, I can force the blockmap to be aligned with my grid:
This is using Boom as the map format and ZenNode as the node builder, in case it matters. I tested multiple levels in a handful of source ports - every boom-compatible level follows the rule, even when launched in GZDoom or Eternity. I tested Paradise and Heartland in their respective ports, they seemed to have blockmaps that are perfectly aligned with the bottom-left most corner of the map.
Thanks for reading, and happy mapping!