Spacer blocks off the adjacent box sounds smart until the boxes themselves aren't set plumb or square, which is half the old kitchens I tear out. Then you're just stacking one error on top of another and the gaps look fine at the top rail but wander at the bottom. Chasing a twisted floor with cabinet reveals is a losing game anyway, because that floor is going to keep moving with the seasons and your 1/16 inch won't hold. Shim and level the boxes first, then the face frame becomes the honest reference again.

Yeah that "6 parts down to zero" line is what got me. I used to think soft jaws were just for fussy tolerance work but honestly the real win is not having to re-clamp. Once you cut them once you can just pop them back on next time and that first setup basically pays for itself forever. The only reason I ever fought it was being too stubborn to stop and make them in the first place. Now I keep a few blanks pre-drilled on the shelf so it's like a 10 minute job when I need a new set.