This is no more:
I don’t know exactly when it changed, but in KiCad V5.1.10 it only shows one “main” unit, and the other is shown in between parenthesis. Switching is done with the in and mm icons in the toolbar on the left:
For designing PCB’s I mostly use metric, except for THT footprints which I place on a 2.54mm grid for breadboard compatibility.
And indeed. switch between grid sizes. Using a coarse grid to draw the PCB outline, mounting holes and such helps a lot with aligning them in rectangles.
iabarry is also right about warning about the grid in Eeschema. Eeschema relies on perfect coordinates for accepting connections between wires and pins of schematic symbols. and this can only be done on a coarse grid. If the connection is correct, then the circle from the pin disappears, and the square at the wire end also disappears.
This is a good connection:
This pin is not connected to the wire:
And when you run Eeschema / Inspect / Electrical Rules Checker then it also tells you so with Pin not connected errors.
This is also why I like the No Connect flag so much. If you forget to make some connection, then ERC alerts you to that fact.